bpo-44206: Add a version number to dictionary keys (GH-26333)

* Store log2(size) instead of size in dict-keys.

* Use enum instead of function pointer to record kind of keys.

* Add version number to dict keys.

Include/cpython/dictobject.h

@@ -82,3 +82,7 @@ typedef struct {
 
 PyAPI_FUNC(PyObject *) _PyDictView_New(PyObject *, PyTypeObject *);
 PyAPI_FUNC(PyObject *) _PyDictView_Intersect(PyObject* self, PyObject *other);
+
+/* Gets a version number unique to the current state of the keys of dict, if possible.
+ * Returns the version number, or zero if it was not possible to get a version number. */
+uint32_t _PyDictKeys_GetVersionForCurrentState(PyDictObject *dict);