Fixity resolution improvements

[NeilMitchell2]

Two related improvements:

1) If a parse results in a fixity error it should be reported through ParseError?, not by raising error.

2) The applyFixities function is very limited. A better signature would be:

applyFixities :: a -> ([FixityError?],a)

Now when there is a fixity error you get back a list of the errors, and a revised document with as many fixities resolved as possible. This would be particularly useful for HLint: http://code.google.com/p/ndmitchell/issues/detail?id=302

And more generally, it should suit any user of HSE. With a simple FixityError? -> ParseError? function the apply fixities could easily be reused for the main parsing.