Blender Git Loki

Add GHash/GSet pop() feature.

Behavior is similar to python's set.pop(), it removes and returns a 'random' entry from the hash.

Notes:
* Popping will return items in same order as ghash/gset iterators (i.e. increasing
order in internal buckets-based storage), unless ghash/gset is modified in between.
* We are keeping a track of the latest bucket we popped out (through a 'state' parameter),
this allows for similar performances to iterators when iteratively popping a whole hash
(without it, we are roughly O(n!), with it we are roughly O(n)...).

Reviewers: campbellbarton

Differential Revision: https://developer.blender.org/D1808