Michael Sloan authored Sep 14, 2018 and Krzysztof Gogolewski committed Sep 14, 2018

Summary:
This adds TH support for the ImplicitParams and RecursiveDo extensions.

I'm submitting this as one review because I cannot cleanly make
the two commits independent.

Initially, my goal was just to add ImplicitParams support, and
I found that reasonably straightforward, so figured I might
as well use my newfound knowledge to address some other TH omissions.

Test Plan: Validate

Reviewers: goldfire, austin, bgamari, RyanGlScott

Reviewed By: RyanGlScott

Subscribers: carter, RyanGlScott, thomie

GHC Trac Issues: #1262

Differential Revision: https://phabricator.haskell.org/D1979