Implement typechecker plugins

Summary:
See https://ghc.haskell.org/trac/ghc/wiki/Plugins/TypeChecker

This is based on work by Iavor Diatchki and Eric Seidel.

Test Plan: validate

Reviewers: simonpj, austin

Reviewed By: austin

Subscribers: gridaphobe, yav, thomie, carter

Differential Revision: https://phabricator.haskell.org/D489

Conflicts:
	docs/users_guide/7.10.1-notes.xml