bpo-36142: Add _PyPreConfig_SetAllocator() (GH-12187)

* _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".