Currently BINARY_SUBSCR_DICT just calls PyDict_GetItemWithError.

We should make use of dk_version for zero cost lookups on dictionaries.

Main problem I can think of:
Dictionaries used as a cache might frequently add and delete keys, so this would perform badly as deoptimizations would be frequent.

Solution: a modified function called PyDict_GetItemWithErrorCached. That takes in our _PyBinarySubscrCache and modifies that. On failure, instead of deoptimizing, just update the cache and go through the usual PyDict_GetItemWithError path. This isn't any slower than going through the unspecialized BINARY_SUBSCR and PyObject_GetItem routine.

I'm going to work on this at the sprint.
CC @markshannon and @iritkatriel.