Move unsat core names to smt engine (#1192)

Move unsat core names to smt engine (#1192)

* Move unsat core names to SmtEnginePrivate. Adds a SetExpressionNameCommand to do this. Removes the names field from GetUnsatCoreCommand.

* Comment

* Pass expression names by reference.

* Update throw specifiers.

* Minor

* Switch expression names to CDMap, simplify interface for printing unsat core names.

* Revert throw specifier change.

* Minor simplifcations