WIP: Generic union, disjoint union CMRAs for sets.

The union and disjoint union CMRAs in algebra/sets and algebra/coPset were originally virtually identical. Version skew set in and coPset's copy of the code didn't receive as much attention as gset's copy.

This MR defines these CMRAs generically in algebra/sets. It leaves the gset and coPset files in place, with notation for backwards compatibility. (When !378 (merged) lands, I slightly favor deprecating the notation.) The MR also supports other set implementations from stdpp.