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.

Support 1-dim memory view types in type annotations (requires support in SliceIndexNode, where we previously only had it in IndexNode).

Avoid C division by 0 to provoque a runtime error in a test, because MSVC considers it a compile time error.

Fix constant folding of multiplied string literals when they appear in another constant expression. In this case, the multiplied value was lost and replaced by the original value.

Try to call "__rmod__()" during string %-formatting when the right side is a string subtype that implements it.
See https://bugs.python.org/issue28598