Simplify 'ExtBits' in the lexer

Authored by harpocrates on Thu, Nov 22, 1:40 PM.

Description

Simplify 'ExtBits' in the lexer

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