The Backpack patch.
Summary: This patch implements Backpack for GHC. It's a big patch but I've tried quite hard to keep things, by-in-large, self-contained. The user facing specification for Backpack can be found at: https://github.com/ezyang/ghc-proposals/blob/backpack/proposals/0000-backpack.rst A guide to the implementation can be found at: https://github.com/ezyang/ghc-proposals/blob/backpack-impl/proposals/0000-backpack-impl.rst Has a submodule update for Cabal, as well as a submodule update for filepath to handle more strict checking of cabal-version. Signed-off-by: Edward Z. Yang <email@example.com> Test Plan: validate Reviewers: simonpj, austin, simonmar, bgamari, goldfire Subscribers: thomie, mpickering Differential Revision: https://phabricator.haskell.org/D1482