Fixed handling of infix operators in types:
- Pretty printing didn't take nested infix operators into account
- Explicit parenthesis were ignored in the fixity parser:
  * I added a constructor `HsParTy' to `HsType' (in the spirit of `HsPar' in
    `HsExpr'), which tracks the use of explicit parenthesis
  * Occurences of `HsParTy' in type-ish things that are not quite types (like
    context predicates) are removed in `ParseUtils'; all other occurences of
    `HsParTy' are removed during type checking (just as it works with `HsPar')