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
We could likely add much more, but those already covers basic behavior and should be able
to catch most errors when editing this code.
Also added some performances tests as well (timing ghash insert/lookup under heavy loads,
for different kinds of keys).
