Replace Expr-level datatype with Node-level DType (#4875)

Replace Expr-level datatype with Node-level DType (#4875)

This PR makes two simultaneous changes:

The new API uses Node-level DType instead of Expr-level Datatype. This required converting 2 of the last remaining calls in the parser that used Expr to use the new API.
Internally constructed datatypes (e.g. for sygus) use Node-level DType instead of Expr-level Datatype. Note this required moving a block of code for handling a corner case of sygus from Datatype -> DType.
This PR removes :

ExprManger::mkDatatypeType.
The Expr-level datatype itself. This PR removes all references to its include file.
It also updates one remaining unit test from Expr -> Node.

This PR will enable further removal of other datatype-specific things in the Expr layer.