gh-101659: Isolate "obmalloc" State to Each Interpreter (gh-101660)

This is strictly about moving the "obmalloc" runtime state from
`_PyRuntimeState` to `PyInterpreterState`.  Doing so improves isolation
between interpreters, specifically most of the memory (incl. objects)
allocated for each interpreter's use.  This is important for a
per-interpreter GIL, but such isolation is valuable even without it.

FWIW, a per-interpreter obmalloc is the proverbial
canary-in-the-coalmine when it comes to the isolation of objects between
interpreters.  Any object that leaks (unintentionally) to another
interpreter is highly likely to cause a crash (on debug builds at
least).  That's a useful thing to know, relative to interpreter
isolation.

Python/sysmodule.c

@@ -1871,7 +1871,9 @@ static Py_ssize_t
 sys_getallocatedblocks_impl(PyObject *module)
 /*[clinic end generated code: output=f0c4e873f0b6dcf7 input=dab13ee346a0673e]*/
 {
-    return _Py_GetAllocatedBlocks();
+    // It might make sense to return the count
+    // for just the current interpreter.
+    return _Py_GetGlobalAllocatedBlocks();
 }
 
 /*[clinic input]