Support for all-star predicates and mutators

[cronburg]

We'll eventually want ATNs which support predicates and mutators on the epsilon edges which fire in a monadic environment so that we can extend the all-star algorithm to support backtracking over mutators with side effects.

The antlr-haskell code already supports parsing of predicates in a g4 grammar definition, but all the parsers I've written currently ignore them.

[slasser]

I've updated my ATN representation so that it supports predicated grammars (see the ATNEdge and ATNEdgeLabel types), although the parsing algorithm itself doesn't do anything with the predicates yet.