Proposed changes to typechecker plugins API

Hi devs, I thought I should flag up some proposed changes relating to typechecker plugins, which Christiaan, Iavor and I have been discussing. The quick summary: * make it possible for plugins to create constraints (Phab:D909); * make it easier for plugins to define special type families; * embed CoreExpr in EvTerm. For more details, see the wiki page: https://ghc.haskell.org/trac/ghc/wiki/Plugins/TypeChecker#Post-7.10changesto... Questions/review/comments very welcome. Adam -- Adam Gundry, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/