Blender Git Loki

Remove public GHash API for union/inter/diff/symmdiff.

This is dead easy to add back when we want (or use other naming if we really
like them better), core code remains exactly the same anyway.

Note we keep checks (isdisjoint, isequal, etc.) for ghashes though, those are
pure-keys non-modifying operations anyway, so here set ops for ghash are
definitively valid (equivalent of set ops on dict.keys() views in py).