Parse Parens

I've changed the parser so:

1. we **need not** write so many parenthesis, e.g. at function applications. 
2. we **cannot** use the old 'p ? e1 : e2' format, and **must** use 'if p then e1 else e2' instead.

All tests pass on goto. This is linked to a corresponding update in liquid-fixpoint

See the diff below e.g. 'include/GHC/List.lhs' to see the new format. In brief we can write specs like:
 
    len v = len x + 1

or 

    len v = 2 * len x - 1