Move Datatype management to ExprManager (#3568)

Move Datatype management to ExprManager (#3568)

This is further work towards decoupling the Expr layer from the Node layer.

This commit makes it so that ExprManager does memory management for Datatype while NodeManager maintains a list of DType.

As a reminder, the ownership policy (and level of indirection through DatatypeIndex) is necessary due to not being able to store Datatype within Node since this leads to circular dependencies in the Node AST.