Consider replacing `ChangedOnly` by more composable predicates

[llucax]

What's needed?

It still seems hard to justify the special case, in particular because if we add more simple predicates it could be really easy to get ChangedOnly while allowing to create more types of predicates more easily.

Specially once #354 and #355 are implemented.

Proposed solution

If we add these:

def is_equal(prev_message: object, new_message: object) -> bool:
    """Return whether `prev_message` is equal to `new_message`."""
    return prev_message == new_message


def is_same(prev_message: object, new_message: object) -> bool:
    """Return whether `prev_message` is the same instance as `new_message`."""
    return prev_message is new_message

Then ChangedOnly is basically is drop_while(is_equal), which it is a bit more verbose, but actually makes it more obvious what's really going on.

[llucax]

We will actually remove ChangedOnly and not add any new predicates, they are just too trivial to be worth adding.