Use an inlined version of PyType_HasFeature() instead of directly accessing "tp_flags" in the generated code.

See https://github.com/cython/cython/issues/3625

The names of Cython's internal types (functions, generator, coroutine, etc.) are now prefixed with the shared module name, instead of making them look like homeless builtins.
See https://bugs.python.org/issue20204
See https://github.com/python/cpython/commit/490055a1673b524da2ebe2312f072aba2a826036

See https://github.com/cython/cython/pull/3616

When test_assert_path_exists or test_fail_if_path_exists
was used on a function containing a CompilerDirectivesNode
it was inherited by that CompilerDirectivesNode. Therefore
you got misleading test failures if the path was in the
function but not within that CompilerDirectivesNode.

* Fix #3588: Make existing overflow code safe

Signed overflow is undefined behaviour in C and compilers can and do optimized on that basis.

* Speed up our overflow impls

Note this is primarily based on performance of compilers which do not
support __builtin_add_overflow (i.e., not Clang >= 3.4 & gcc >= 5),
mostly looking at several gcc 4 releases (used by older, supported,
RHEL releases and Debian 8 "Jessie") and MSVC.

* Use __builtin_XXX_overflow if available

This is much quicker in general, as these all just then read the overflow flag from the status register.

Disable GC during hacked calls to PyType_Ready() that simulate a heap type for an actual non-heap type, because it can lead to crashes if GC visits such a hacked type.
Closes https://github.com/cython/cython/issues/3603

Disable GC during hacked calls to PyType_Ready() that simulate a heap type for an actual non-heap type, because it can lead to crashes if GC visits such a hacked type.
Closes https://github.com/cython/cython/issues/3603

Reduce the code overhead of the memoryview implementation a little by disabling CyFunctions for its methods.

Reduce the code overhead of exception raising in generated Cython code and the memoryview code by avoiding explicit calls to create the exception and prepared constant argument tuples. "raise Exc, message" can do this implicitly.

Move less common ref-counting cases for memory views out of the fast-path and only look up the atomic acquisition count value once per INC/DEC operation.

When it's merged into the main module, wrap it in a CompilerDirectivesNode.