Exhaustiveness checking for pattern synonyms

Hello devs, I have implemented exhaustiveness checking for pattern synonyms. The idea is very simple, you specify a set of pattern synonyms (or data constructors) which are regarded as a complete match. The pattern match checker then uses this information in order to check whether a function covers all possibilities. Specification: https://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms/CompleteSigs https://phabricator.haskell.org/D2669 https://phabricator.haskell.org/D2725 Matt