Add a hash for faster look-ups on collection->gobject,
This avoids a full list lookup for every object added via Python's
CollectionObject.link as well as linking via BKE_collection_object_add_*
functions.
While the speedup is non-linear, linking & unlinking 100k objects from
Python is about 50x faster. Although unlinking all objects in order
(a best-case for linked lists) is approximately the same speed.
Ref !104553.
ea97bb1641