Add uext() and sext() builder methods to ArgumentType.
Add uext() and sext() builder methods to ArgumentType.

This makes it simpler to construct arguments like:

  ArgumentType::new(I32).uext()

stoklund/cretonne,stoklund/cretonne,sunfishcode/cretonne,sunfishcode/cretonne,stoklund/cretonne,sunfishcode/cretonne