Somehow ad hoc descriptions don't work anymore for grpcio (probably
because the README.rst isn't at the distribution root for grpcio).

Before, Cython would *need* to be imported immediately if generated
files did not already exist. Now, missing generated files will trigger a
`setup_requires` inclusion of Cython and defer cythonization until
extension build-time. If cythonization was specified via environment
variable and setup could not find Cython, the extensions are poisoned
instead of blocking non-extension commands from running.

Before we patched the link command, now we just patch `spawn` as an
updatable catch-all solution to ARG_MAX limitations on bash for MSYS and
MinGW and friends.

Removes MinGW flags from various build scripts, since they're now set in
our `setup.py`s by default. Also removes the arguments for the
extensions in grpcio's setup.py; that function was already accessing
globals anyway, might as well go whole-hog.

Before we patched the link command, now we just patch `spawn` as an
updatable catch-all solution to ARG_MAX limitations on bash for MSYS and
MinGW and friends.

The modified link command was originally taken from a Python 2.x distutils.

Even if GRPC_PYTHON_BUILD_WITH_CYTHON is not specified, if the files are
not present then we will fall back to generating with Cython. This
relegates GRPC_PYTHON_BUILD_WITH_CYTHON to providing a regeneration
option rather than being a necessary build environment variable.

Moves all tests into a separate package. This does not change existing
supported means of running tests (e.g. through run_tests.py).