Refactor type ascriptions in the parser (#3825)

Refactor type ascriptions in the parser (#3825)

Towards parser migration.

This consolidates two blocks of code (cvc/smt2) that do type ascriptions to a utility function in the parser. It updates this function to use the new API.

This code will be further refactored when the interface for parametric datatype constructors is further developed in the new API.