fix requiredHoles to return symbols directly

Commit https://github.com/vjovanov/mpde/commit/5562a5af3156ac7ce88c4f53b6f226987f7247c5 introduced sorted symbols in the YYTransformer, so requiredHoles was expected to return symbol ids instead of hole ids as before. However, because of reflection the symbol ids weren't accessible in the dsl even though a list of symbols was provided to requiredHoles. With this commit, requiredHoles returns the symbols directly, using the hole ids as index into the provided list of hole symbols. The ugly type casting is done by the YYTransformer.