Drop GHC 8.2 compatibility

Authored by RyanGlScott on Oct 3 2018, 7:41 AM.

Description

Drop GHC 8.2 compatibility

Summary:
GHC 8.6.1 is out, so now GHC's support window only extends
back to GHC 8.4. This means we can delete gobs of code that were
only used for GHC 8.2 support. Hooray!

Test Plan: ./validate

Reviewers: bgamari, Phyx, erikd

Reviewed By: bgamari, Phyx

Subscribers: rwbarton, erikd, carter

Differential Revision: https://phabricator.haskell.org/D5192