fix imported identifiers in maps

[hayes]

fixes #173

Currently when resolving const values, identifiers are not resolved recursively, so if identifiers nested inside other ast nodes (eg maps) are used, the may not have been transformed correctly, which breaks referencing types in other files in these cases.

[hayes]

@kevin-greene-ck could you take a look at this when you have a chance?

[niodice]

cc @kevin-greene-ck (tagging you since you seem to be the most active contributor). Do you think this is something suitable for merging? I am also hitting this issue as a blocker for adopting this package for thrift file compilation.