Obtaining a DynFlags is difficult, making using the lexer/parser
for pure parsing/lexing unreasonably difficult, even with mkPStatePure.
This is despite the fact that we only really need
- language extension flags
- warning flags
- a handful of boolean options
The new mkParserFlags' function makes is easier to directly construct a
ParserFlags. Furthermore, since pExtsBitmap is just a footgun, I've gone
ahead and made ParserFlags an abstract type.