Alec Theriault authored Nov 22, 2018 and Ben Gamari committed Nov 22, 2018

The main change is to export 'ExtBits' instead of defining/exporting a
bunch of boilerplate functions that test for a particular 'ExtBits'.
In the process, I also

  * cleaned up an unneeded special case for 'ITstatic'
  * made 'UsePosPrags' another variant of 'ExtBits'
  * made the logic in 'reservedSymsFM' match that of 'reservedWordsFM'

Test Plan: make test

Reviewers: bgamari, alanz

Subscribers: sjakobi, rwbarton, mpickering, carter

Differential Revision: https://phabricator.haskell.org/D5332