Quantified constraints

Friends Now that the Quantified Constraints proposalhttps://github.com/Gertjan423/ghc-proposals/blob/quantified-constraints/prop... is accepted, I want to commit it to GHC before the 8.6 release. It's up on https://phabricator.haskell.org/D4724 for your review. It's rebased on master, and validates clean. There's also a useful wiki pagehttps://ghc.haskell.org/trac/ghc/wiki/QuantifiedConstraints. Thanks! Simon