Make `singleConstructor` cope with pattern synonyms

Previously, `singleConstructor` didn't handle singleton `COMPLETE` sets
of a single pattern synonym, resulting in incomplete pattern warnings.

This is fixed by making `singleConstructor` (now named
`singleMatchConstructor`) query `allCompleteMatches`, necessarily making
it effectful. As a result, most of this patch is concerned with
threading the side-effect through to `singleMatchConstructor`.