tidy up union

[petrelharp]

A few things:

• there's a note that says we can remove the restriction that the non-shared bits of other not be above the shared bit. I think we can do this now, since we're sorting on mutation time.
• I don't think that we're checking whether the same value appears more than once in node_mapping, and I suspect this could make bad things happen - but, maybe it's ok?