This moves from `GHash` to the type-safe and better-performing `Map`.
It uses the construct on first use idiom to simplify creation and freeing
of the map, removing the need to register an at-exit callback.
The change was initially motivated by seeing the hash lookups in a profile
of drawing a large node tree. Though my testing wasn't exactly scientific,
I did observe a few percent performance improvement with this change.
Pull Request: https://projects.blender.org/blender/blender/pulls/129954
c709b59539