Blender Git Loki

Generalization of function caching to avoid unnecessary recompilation.

The function cache allows callers to store functions in a cache (hash table).
The functions also have a reference count, so replacing the function
in the cache (when the node tree changes) does not immediately delete
the current instance. This allows background threads to keep using the
old function until they can cleanly return (tasks should be restarted
when the nodes become outdated, but this is up to the caller system).