WIP on combined Step 1 and 3 for Trees That Grow, HsExpr
See https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow Trees that grow extension points are added for - HsExpr Updates haddock submodule Test Plan: ./validate Reviewers: bgamari, goldfire Subscribers: rwbarton, thomie, shayan-najd, mpickering Differential Revision: https://phabricator.haskell.org/D4177