Consider having Eras not complete eagerly

[jasongrlicky]

Right now Eras complete eagerly - that is, if you have .called_any() on an Expectation, for era completion purposes, it will behave as if it is a .called_once().

This isn't intuitive - we should accept any number of calls of that function before considering the era finished.

The end goal would be that something like the following test should pass:

#[test]
fn test_eras_do_not_complete_eagerly() {
    let mut e = Expectations::new();

    // Expectations
    e.expect::<(), ()>("c").called_any();
    e.then();
    e.expect::<(), ()>("d").called_once();
        
    // Calls
    e.was_called::<(), ()>("c", ()); // Does NOT complete the first era, unlike today
    e.was_called::<(), ()>("c", ()); // Completes first era
    e.was_called::<(), ()>("d", ()); // Completes second era
}

That said, we'll need to come up with a good solution for the following cases:

• What happens when a called_any() is followed by a called_any() for the same method in the next era?

We may also need to rethink the way that Eras operate fundamentally. We could possibly also use this as an opportunity to create global expectations, or allow return/modifying behaviors to be set on a method level?