Expose a choice field for SemanticValues in dictionaries

[joankaradimov]

I notice that the Trie behaves more like a set and its match method returns the match length. Would it be viable to make it behave like a map with its match returning the choice index (that can be kept as an additional field in the Info struct).

I think that would make for a more consistent interface. And allow for more performant grammar handling in certain cases.

[yhirose]

Sounds good.