Fix a threading.local() and a unicode type analysis bug and add support for round(f, 0)

The GC did not visit the thread state dict, which made it forget it's entries after a gc collection

Including the ability to set __class__

Previously there was no way to reliably run the default setattr
behavior, which can be requested by calling object.__setattr__
or PyObject_GenericSetattr.  When those were called, we would
check to see if there is a custom __setattr__ defined and then
try to call that.  But users can and do define custom __setattr__
functions that defer to object.__setattr__, so we need a way to
run the default behavior without deferring to a custom setattr.

Make dict.fromkeys a class method

I think these are getting accessed but not used right now

2-arg iter() variant
bytearray(xrange(256))
better support for multiple inheritance from both old- and new-style classes
set.pop()
str.rindex
set.discard

__setattr_ and thread._local

(scratch space is pre-allocated stack space since the IC can't
allocate new space itself, in order to have unwinding work)

Allocate a fixed amount (currently: 40 bytes) of extra stack space
in our runtime ics.  This involves changing the function prologue+
epilogue to do more rsp adjustment, and modifying the .eh_frame sections
we generate.

One tricky thing is that we currently use frame pointer elimination
in our runtime ics, but the rest of the scratch space logic had assumed
the scratch would be rbp-relative, which I had to convert to rsp-relative.

This should help with sys_argv breaking.
Hopefully it doesn't introduce too many re-calculations of the
CPython expected output.

In particular, subclassing from (oldstyle_cls, newstyle_cls)
results in a newstyle class, since ClassType.__new__ defers
to any of the bases that are not oldstyle classes.

I think this is ok, but it's implemented pretty differently from CPython
for now so I'm not sure.

hacky (same as the non-iter version) but seems good enough for now

This will prevent tests from dirtying the source/build directories,
or worse overwriting/removing files we care about.

This commit expands on some existing hacks, which I don't like, but
they should hopefully be going away soon.

add cpython buffer + sqlite3 module, and a couple tests.

Implement further changes required for virtualenv

don't assert() if python errors inside of a CFGVisitor's RAII scope

INSTALLING.md: add instructions on getting gdb pretty-printers to work