This introduces new methods to check the results of function calls. It

introduces returned() (for checking return values), raised() (for

checking raised exception types), raised_with_message() (for checking

raised exception types and exception messages), and last_* variations

of each.
It also brings all call checks (called_with(), returned(),

raised(), and raised_with_message()) to SpyCall, giving callers a

way to more conveniently determine results from individual calls.

Unit tests pass.