[3.7] bpo-36389: _PyObject_IsFreed() now also detects uninitialized memory (GH-12770...

[3.7] bpo-36389: _PyObject_IsFreed() now also detects uninitialized memory (GH-12770) (GH-12788)

* bpo-36389: _PyObject_IsFreed() now also detects uninitialized memory (GH-12770)

Replace _PyMem_IsFreed() function with _PyMem_IsPtrFreed() inline
function. The function is now way more efficient, it became a simple
comparison on integers, rather than a short loop. It detects also
uninitialized bytes and "forbidden bytes" filled by debug hooks
on memory allocators.

Add unit tests on _PyObject_IsFreed().

(cherry picked from commit 2b00db68554422ec37faba2a80179a0172df6349)

* bpo-36389: Change PyMem_SetupDebugHooks() constants (GH-12782)

Modify CLEANBYTE, DEADDYTE and FORBIDDENBYTE constants: use 0xCD,
0xDD and 0xFD, rather than 0xCB, 0xBB and 0xFB, to use the same byte
patterns than Windows CRT debug malloc() and free().

(cherry picked from commit 4c409beb4c360a73d054f37807d3daad58d1b567)