We're now only ~2x slower, instead of 10x.

Based on #755.

A comment in the code goes into detail about the timing. Here it is
again:

 Timings taken Feb 21 2018 prior to integration of #755
 python -m perf timeit -s 'import gevent' 'gevent.Greenlet()'
 3.6.4       : Mean +- std dev: 1.08 us +- 0.05 us
 2.7.14      : Mean +- std dev: 1.44 us +- 0.06 us
 PyPy2 5.10.0: Mean +- std dev: 2.14 ns +- 0.08 ns

 After the integration of spawning_stack, spawning_greenlet,
 and spawn_tree_locals on that same date:
 3.6.4       : Mean +- std dev: 8.92 us +- 0.36 us ->  8.2x
 2.7.14      : Mean +- std dev: 14.8 us +- 0.5 us  -> 10.2x
 PyPy2 5.10.0: Mean +- std dev: 3.24 us +- 0.17 us ->  1.5x

Selected bench_spawn output on 3.6.4 before:

//gevent36/bin/python src/greentest/bench_spawn.py eventlet --ignore-import-errors
using eventlet from //gevent36/lib/python3.6/site-packages/eventlet/__init__.py
spawning: 11.93 microseconds per greenlet
sleep(0): 23.49 microseconds per greenlet

//gevent36/bin/python src/greentest/bench_spawn.py gevent --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 3.39 microseconds per greenlet
sleep(0): 17.59 microseconds per greenlet

//gevent36/bin/python src/greentest/bench_spawn.py geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 8.71 microseconds per greenlet

//gevent36/bin/python src/greentest/bench_spawn.py geventraw --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 2.09 microseconds per greenlet

//gevent36/bin/python src/greentest/bench_spawn.py none --ignore-import-errors
    noop: 0.33 microseconds per greenlet

And after:

//gevent36/bin/python bench_spawn.py gevent --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 12.99 microseconds per greenlet -> 3.8x

//gevent36/bin/python bench_spawn.py geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 19.49 microseconds per greenlet -> 2.2x

//gevent36/bin/python bench_spawn.py geventraw --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 4.57 microseconds per greenlet -> 2.2x

We're approximately the speed of eventlet now.

Refs #755

Current numbers on my workstation:

CPython 3.6.4

//gevent36/bin/python bench_spawn.py eventlet --ignore-import-errors
using eventlet from //python3.6/site-packages/eventlet/__init__.py
spawning: 11.93 microseconds per greenlet
sleep(0): 23.49 microseconds per greenlet

//gevent36/bin/python bench_spawn.py gevent --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 3.39 microseconds per greenlet
sleep(0): 17.59 microseconds per greenlet

//gevent36/bin/python bench_spawn.py geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 8.71 microseconds per greenlet
sleep(0): 16.99 microseconds per greenlet
 joining: 7.34 microseconds per greenlet

//gevent36/bin/python bench_spawn.py geventraw --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 2.09 microseconds per greenlet
sleep(0): 13.57 microseconds per greenlet

//gevent36/bin/python bench_spawn.py none --ignore-import-errors
    noop: 0.33 microseconds per greenlet

//gevent36/bin/python bench_spawn.py --with-kwargs eventlet --ignore-import-errors
using eventlet from //python3.6/site-packages/eventlet/__init__.py
spawning: 12.06 microseconds per greenlet
sleep(0): 24.25 microseconds per greenlet

//gevent36/bin/python bench_spawn.py --with-kwargs gevent --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 4.62 microseconds per greenlet
sleep(0): 22.18 microseconds per greenlet

//gevent36/bin/python bench_spawn.py --with-kwargs geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 10.25 microseconds per greenlet
sleep(0): 18.89 microseconds per greenlet
 joining: 7.14 microseconds per greenlet

//gevent36/bin/python bench_spawn.py --with-kwargs geventraw --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 3.27 microseconds per greenlet
sleep(0): 15.03 microseconds per greenlet

//gevent36/bin/python bench_spawn.py --with-kwargs none --ignore-import-errors
    noop: 0.50 microseconds per greenlet

CPython 2.7.14

//gevent27/bin/python bench_spawn.py eventlet --ignore-import-errors
using eventlet from //python2.7/site-packages/eventlet/__init__.pyc
spawning: 17.84 microseconds per greenlet
sleep(0): 31.42 microseconds per greenlet

//gevent27/bin/python bench_spawn.py gevent --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 4.95 microseconds per greenlet
sleep(0): 21.40 microseconds per greenlet

//gevent27/bin/python bench_spawn.py geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 12.44 microseconds per greenlet
sleep(0): 22.90 microseconds per greenlet
 joining: 10.68 microseconds per greenlet

//gevent27/bin/python bench_spawn.py geventraw --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 2.33 microseconds per greenlet
sleep(0): 15.11 microseconds per greenlet

//gevent27/bin/python bench_spawn.py none --ignore-import-errors
    noop: 0.58 microseconds per greenlet

//gevent27/bin/python bench_spawn.py --with-kwargs eventlet --ignore-import-errors
using eventlet from //python2.7/site-packages/eventlet/__init__.pyc
spawning: 19.38 microseconds per greenlet
sleep(0): 31.62 microseconds per greenlet

//gevent27/bin/python bench_spawn.py --with-kwargs gevent --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 6.59 microseconds per greenlet
sleep(0): 23.12 microseconds per greenlet

//gevent27/bin/python bench_spawn.py --with-kwargs geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 13.78 microseconds per greenlet
sleep(0): 21.65 microseconds per greenlet
 joining: 10.86 microseconds per greenlet

//gevent27/bin/python bench_spawn.py --with-kwargs geventraw --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 3.59 microseconds per greenlet
sleep(0): 16.14 microseconds per greenlet

//gevent27/bin/python bench_spawn.py --with-kwargs none --ignore-import-errors
    noop: 0.65 microseconds per greenlet

PyPy2 5.10.0

//geventpypy/bin/python bench_spawn.py eventlet --ignore-import-errors
using eventlet from //geventpypy/site-packages/eventlet/__init__.pyc
spawning: 29.91 microseconds per greenlet
sleep(0): 84.24 microseconds per greenlet

//geventpypy/bin/python bench_spawn.py gevent --ignore-import-errors
using gevent from //src/gevent/__init__.py
spawning: 0.73 microseconds per greenlet
sleep(0): 33.02 microseconds per greenlet

//geventpypy/bin/python bench_spawn.py geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 2.49 microseconds per greenlet
sleep(0): 38.56 microseconds per greenlet
 joining: 13.08 microseconds per greenlet

//geventpypy/bin/python bench_spawn.py geventraw --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 0.52 microseconds per greenlet
sleep(0): 29.63 microseconds per greenlet

//geventpypy/bin/python bench_spawn.py none --ignore-import-errors
    noop: 0.02 microseconds per greenlet

//geventpypy/bin/python bench_spawn.py --with-kwargs eventlet --ignore-import-errors
using eventlet from //geventpypy/site-packages/eventlet/__init__.pyc
spawning: 30.36 microseconds per greenlet
sleep(0): 83.00 microseconds per greenlet

//geventpypy/bin/python bench_spawn.py --with-kwargs gevent --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 1.56 microseconds per greenlet
sleep(0): 34.80 microseconds per greenlet

//geventpypy/bin/python bench_spawn.py --with-kwargs geventpool --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 3.48 microseconds per greenlet
sleep(0): 39.43 microseconds per greenlet
 joining: 16.21 microseconds per greenlet

//geventpypy/bin/python bench_spawn.py --with-kwargs geventraw --ignore-import-errors
using gevent from //src/gevent/__init__.pyc
spawning: 1.51 microseconds per greenlet
sleep(0): 52.19 microseconds per greenlet

//geventpypy/bin/python bench_spawn.py --with-kwargs none --ignore-import-errors
    noop: 0.13 microseconds per greenlet

[skip ci]

Make the check_callback do stuff under PyPy to trigger signal deliver…

Cleanup the libev destroyed data like the libuv destroyed data.

Fix race conditions in libuv child watchers.

(hopefully) fix a crash destroying loops in test__core.py; only seen on Windows, but with some tweaks I could reproduce it locally.

Fixes #1104

Be more careful about issuing the SSL warning on Py2.

Also some other doc updates.

[skip ci]

Fixes #1108.

Update c-ares to 1.14. Fixes #1105.

Support GEVENT_RESOLVER_SERVERS for configuring dnspython nameservers.

Also allow this for ares, and deprecate the GEVENTARES_SERVERS environment variable.

Fixes #1103.

ccache saves us time on compiling, but not linking. The difference is unlikely to be significant for test purposes.

Make socket.sendall() fast on PyPy3: 60MB/s -> 600MB/s

I see in on OS X even without gevent involved. I see it with gevent involved both on the sendall branch and the master branch. I see it on Travis (now) with the sendall patch.
I don't know if it's been seen before the sendall patch on Travis---flaky builds tend to get rerun. None of the other SSL tests are failing, though, and the pypy version of test_ssl.py notes that test_handshake has been removed from CPython altogether.
It looks like it's very dependent on buffering and things like that.

TestEtcHosts round-robin failures also seen on appveyor with the system resolver---because we're not using the local system hosts file.

Share the chunking code between Python 2 and 3.

Also get the tests passing (read: skip a bunch) with 5.10.1 on OS X,
which is apparently less well tested than Linux.

We don't test pypy3 on appveyor, so:

[skip appveyor]

Simplify libev default loop handling and let it regenerate itself.

Don't let destroying the default C loop from two different Python loo…

Don't let destroying the default C loop from two different Python loop objects crash the process. Fixes #1098