gh-112532: Use separate mimalloc heaps for GC objects (gh-113263)

* gh-112532: Use separate mimalloc heaps for GC objects

In `--disable-gil` builds, we now use four separate heaps in
anticipation of using mimalloc to find GC objects when the GIL is
disabled. To support this, we also make a few changes to mimalloc:

* `mi_heap_t` and `mi_tld_t` initialization is split from allocation.
  This allows us to have a `mi_tld_t` per-`PyThreadState`, which is
  important to keep interpreter isolation, since the same OS thread may
  run in multiple interpreters (using different PyThreadStates.)

* Heap abandoning (mi_heap_collect_ex) can now be called from a
  different thread than the one that created the heap. This is necessary
  because we may clear and delete the containing PyThreadStates from a
  different thread during finalization and after fork().

* Use enum instead of defines and guard mimalloc includes.

* The enum typedef will be convenient for future PRs that use the type.
* Guarding the mimalloc includes allows us to unconditionally include
  pycore_mimalloc.h from other header files that rely on things like
  `struct _mimalloc_thread_state`.

* Only define _mimalloc_thread_state in Py_GIL_DISABLED builds

Include/internal/pycore_pystate.h

@@ -187,6 +187,7 @@ extern PyThreadState * _PyThreadState_New(
     int whence);
 extern void _PyThreadState_Bind(PyThreadState *tstate);
 extern void _PyThreadState_DeleteExcept(PyThreadState *tstate);
+extern void _PyThreadState_ClearMimallocHeaps(PyThreadState *tstate);
 
 // Export for '_testinternalcapi' shared extension
 PyAPI_FUNC(PyObject*) _PyThreadState_GetDict(PyThreadState *tstate);