Refactor Commands to use the Public API. (#5105)

Refactor Commands to use the Public API. (#5105)

This is work towards eliminating the Expr layer. This PR does the following:

Replace Expr/Type with Term/Sort in the API for commands.
Remove the command export functionality which is not supported.
Since many commands now call the Solver functions instead of the SmtEngine ones, their behavior may be a slightly different. For example, (get-unsat-assumptions) now only works in incremental mode. In some cases, CVC4 will not recover from non-fatal errors.