Revert "make Coroutine type a non-iterable/iterator and instead let __await__() return a thin Iterator wrapper"

This reverts commit cf51fa4e.

make Coroutine type a non-iterable/iterator and instead let __await__() return a thin Iterator wrapper

make sure Cython's generator and coroutine types are registered with the Generator/Coroutine ABCs also in Py3.5
patch both types also into asyncio (in Py<3.5)

rewrite __Pyx_ReturnWithStopIteration() to fix returning tuples and also fix it on the other side in __Pyx_PyGen_FetchStopIterationValue()

make Cython's Coroutine type and extension types work with Python's async-for loop, implement new slots in Py3.5

Use different mangling prefix in foo_api.h files

Fix links to CEP 516

split Cython's Generator type into separate Coroutine and Generator types to prepare PEP 492 implementation

Progress indicator for cythonize()