Add guards to module dicts
Add guards to some dicts, particularly module dicts (i.e. globals()) so that the specialiser can convert LOAD_GLOBAL into LOAD_CONST. Requirements: 1. No change to size of other (unguarded) dicts. 2. No slow down for reads to any dict. 3. Neglible slow down for writes/deletes to other (unguarded) dicts. 4. Small slow down for writes to guarded dicts (if no guards are triggered).