Kinda decent testing for the parser. It uncovered two errors:

  - Stupid error in the lexer:
      Identifiers starting with `_' ended up as varsyms instead of varids
  - Infix expression parsing was unnecesarily restrictive:
      S $ \x -> x) required parenthesis for the lambda

All fixed now, and we can sucessfully parse a good amount of real Haskell code
(see bluejelly-bjc/src/test/reosurces/parser.tests).