Enable overloading the assignment operator.

Conflicts:
	setup.py

Conflicts:
	CHANGES.rst
	Cython/Shadow.py

rename "gi_running" attribute of coroutine type to "cr_running" and adapt some docstrings to the type split

re-implement Coroutine.__await__() using a dedicated wrapper type to avoid having to make a Coroutine half an Iterator

Fix CFLAGS environ in tox config

Add signbit to math.pxd.

Also reorder a few imports to alphabetical order.

Conflicts:
	Cython/Shadow.py

keep C++ code out of C files and vice-versa by splitting module preamble (original patch by Lars Mans)

exclude two more modules from stdlib compilation that require bootstrapping before importing Cython modules