gh-106931: Intern Statically Allocated Strings Globally (gh-107272)

We tried this before with a dict and for all interned strings.  That ran into problems due to interpreter isolation.  However, exclusively using a per-interpreter cache caused some inconsistency that can eliminate the benefit of interning.  Here we circle back to using a global cache, but only for statically allocated strings.  We also use a more-basic _Py_hashtable_t for that global cache instead of a dict.

Ideally we would only have the global cache, but the optional isolation of each interpreter's allocator means that a non-static string object must not outlive its interpreter.  Thus we would have to store a copy of each such interned string in the global cache, tied to the main interpreter.

Python/hashtable.c

@@ -129,6 +129,13 @@ _Py_hashtable_size(const _Py_hashtable_t *ht)
 }
 
 
+size_t
+_Py_hashtable_len(const _Py_hashtable_t *ht)
+{
+    return ht->nentries;
+}
+
+
 _Py_hashtable_entry_t *
 _Py_hashtable_get_entry_generic(_Py_hashtable_t *ht, const void *key)
 {