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!

Modules/_testcapimodule.c

@@ -4297,7 +4297,7 @@ pymem_malloc_without_gil(PyObject *self, PyObject *args)
 static PyObject*
 test_pymem_getallocatorsname(PyObject *self, PyObject *args)
 {
-    const char *name = _PyMem_GetAllocatorsName();
+    const char *name = _PyMem_GetCurrentAllocatorName();
     if (name == NULL) {
         PyErr_SetString(PyExc_RuntimeError, "cannot get allocators name");
         return NULL;