Check some child nodes against the correct nogil context when they are actually being evaluated in the outer scope (e.g default arguments or annotations of a nogil function).

Discard invisible type annotations from cdef functions after applying them, as they might get in the way of @nogil declarations etc.

Fixes Github issue #2603.

This provides a test case for #2603

Warn when no "language_level" is specified.

Closes #2593.

Also removes the usage of the header file "long.cpp" which was removed in Pythran 0.8.7.

Use exception stack in Py3.7

Disable a failing test case that is less important than the rest of the Py3.7 exception stack implementation.

Always break the frame back-pointer chain when exiting from a generator, so that it can be reconnected on the next entry.

Make sure builtins add their utility code also when they are only being used as names (applies to StopAsyncIteration, for example).

Generally allow the "nogil" decorator and context manager to accept boolean arguments like "@nogil(False)".
See #2579.

Closes #2557.