Eliminate uses of Expr in SmtEngine interface (#5240)

Eliminate uses of Expr in SmtEngine interface (#5240)

This eliminates all use of Expr in the SmtEngine except in a few interfaces (setUserAttribute, getAssignment) whose signature is currently in question.

The next steps will be to (1) eliminate Expr from the smt/model.h interface, (2) replace Type by TypeNode in Sort.