core.pyx: set the EV_* constants from inside the core module.

This has the following advantages:

1. replacing core with another implementation should be easier. I had
   to remove these constants while playing with a libev
   implementation, they use different constants.

2. one can implement deferred imports.

3. the core module is now self contained, i.e. one can "import core" from in

gevent/__init__.py

@@ -38,12 +38,6 @@ del sys
 
 
 from gevent import core
-core.EV_TIMEOUT = 0x01
-core.EV_READ    = 0x02
-core.EV_WRITE   = 0x04
-core.EV_SIGNAL  = 0x08
-core.EV_PERSIST = 0x10
-
 from gevent.greenlet import Greenlet, joinall, killall
 spawn = Greenlet.spawn
 spawn_later = Greenlet.spawn_later

gevent/core.pyx

@@ -30,6 +30,11 @@ from pprint import pformat
 import weakref
 
 cimport levent
+EV_TIMEOUT = levent.EV_TIMEOUT
+EV_READ = levent.EV_READ
+EV_WRITE = levent.EV_WRITE
+EV_SIGNAL = levent.EV_SIGNAL
+EV_PERSIST = levent.EV_PERSIST
 
 import _socket
 gaierror = _socket.gaierror