gh-81057: Move the Allocators to _PyRuntimeState (gh-99217)

The global allocators were stored in 3 static global variables: _PyMem_Raw, _PyMem, and _PyObject.  State for the "small block" allocator was stored in another 13.  That makes a total of 16 global variables. We are moving all 16 to the _PyRuntimeState struct as part of the work for gh-81057.  (If PEP 684 is accepted then we will follow up by moving them all to PyInterpreterState.)

https://github.com/python/cpython/issues/81057