Exhaustiveness checking for pattern synonyms

[Matthew Pickering]

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