bpo-36763: Add PyMemAllocatorName (GH-13387)

* Add PyMemAllocatorName enum
* _PyPreConfig.allocator type becomes PyMemAllocatorName, instead of
  char*
* Remove _PyPreConfig_Clear()
* Add _PyMem_GetAllocatorName()
* Rename _PyMem_GetAllocatorsName() to
  _PyMem_GetCurrentAllocatorName()
* Remove _PyPreConfig_SetAllocator(): just call
  _PyMem_SetupAllocators() directly, we don't have do reallocate the
  configuration with the new allocator anymore!
* _PyPreConfig_Write() parameter becomes const, as it should be in
  the first place!