Adam Gundry authored Feb 14, 2017 and Ben Gamari committed Feb 14, 2017

This implements automatic constraint solving for the new HasField class
and modifies the existing OverloadedLabels extension, as described in
the GHC proposal
(https://github.com/ghc-proposals/ghc-proposals/pull/6). Per the current
form of the proposal, it does *not* currently introduce a separate
`OverloadedRecordFields` extension.

This replaces D1687.

The users guide documentation still needs to be written, but I'll do
that after the implementation is merged, in case there are further
design changes.

Test Plan: new and modified tests in overloadedrecflds

Reviewers: simonpj, goldfire, dfeuer, bgamari, austin, hvr

Reviewed By: bgamari

Subscribers: maninalift, dfeuer, ysangkok, thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D2708