Make region/locset interface more consistent. (#943)

* Add implicit conversions from mlocation and mlocation_list to locset.
* Add implicit conversions from mcable and mcable_list to region.
* Change arguments for `ls::location` to `(branch, position)`.
* Extend unit tests to suit.
* Make locset and region wrapping less promiscuous by demanding that
expression implementation classes derive from locset_tag or region_tag
respectively.