Drop GHC 8.0 compatibility
GHC 8.4.1 is out, so now GHC's support window only extends back to GHC 8.2. This means we can delete gobs of code that were only used for GHC 8.0 support. Hooray! Test Plan: ./validate Reviewers: bgamari, erikd, dfeuer Reviewed By: bgamari, dfeuer Subscribers: alexbiehl, dfeuer, rwbarton, thomie, carter Differential Revision: https://phabricator.haskell.org/D4492
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