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