Fix #9

See #9 for details. There are two related changes:

- provide 'pat' and 'patWithModes' quasiquotations
- use parser instead of Hs.parsePat in qq so that it's possible to explicitely specify a parser to use in pattern contexts.

The second change is incompatible with previous behavior, but I don't expect this to be a problem in practice, because splicing a Syntax tree of a pattern as a pattern does not seem very useful to me.