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().
* _PyPreConfig_Write() now reallocates the pre-configuration with the
new memory allocator.
* It is no longer needed to force the "default raw memory allocator"
to clear pre-configuration and core configuration. Simplify the
code.
* _PyPreConfig_Write() now does nothing if called after
Py_Initialize(): no longer check if the allocator is the same.
* Remove _PyMem_GetDebugAllocatorsName(): dev mode sets again
allocator to "debug".
The development mode now uses the effective name of the debug memory
allocator ("pymalloc_debug" or "malloc_debug"). So the name doesn't
change after setting the memory allocator.
