Blender Git Loki

GHash: Add difference and symmetric_difference (with tests).

Also fix some issues in previous commits.

Notes about our 'symmetric_difference':
* name is taken from py, but our function takes multiple arguments,
and returns keys which are present in one and only one given ghash.
So we may want a better name for that
(real symmetric difference only accepts/works with two args)!
* Its code is a bit more complex than the others, think it's worth it though,
XOR is a really useful operation imho.