WIP: Generic union, disjoint union CMRAs for sets.
The union and disjoint union CMRAs in
algebra/coPset were originally virtually identical. Version skew set in and
coPset's copy of the code didn't receive as much attention as
This MR defines these CMRAs generically in
algebra/sets. It leaves the
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.