[issue46657] Add mimalloc memory allocator

[Christian Heimes]

Christian Heimes <lists at cheimes.de> added the comment:

References:

- C11 stdatomic.h https://en.cppreference.com/w/c/atomic
- mimalloc-atomic.h https://github.com/microsoft/mimalloc/blob/master/include/mimalloc-atomic.h
- MSVC Interlocked Variable Access https://docs.microsoft.com/de-de/windows/win32/sync/interlocked-variable-access

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue46657>
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