.called_once() violations should fail quickly

[asomers]

If an expectation has .called_once yet gets called more than once, it doesn't currently fail until the end of the test. This can be very confusing, especially if the following .returning statement has side effects. It would be best if the test failed immediately on the second call. For example:

    fn times_once() {
        pub trait A {
            fn foo(&self) -> u32;
        }

        create_mock! {
            impl A for AMock (self) {
                expect_foo("foo"):
                fn foo(&self) -> u32;
            }
         }

        let mut mock = AMock::new();
        let mut o = Some(0);
        mock.expect_foo()
            .called_once()
            .returning(move |_| o.take().unwrap());

        mock.foo();
        mock.foo();
    }

This code will fail with the error

panicked at 'called `Option::unwrap()` on a `None` value'

instead of something about foo being called too many times.

[jasongrlicky]

Thanks for reporting this, @asomers!