Add matchers

[bitwizeshift]

As an extension to #7, it'd be nice to have "matcher" logic, which can help simplify testing that values match a given expectation in a simplified way. These matchers can be based off of the way other testing libraries are written, such as Catch2 or GTest.

This should also be usable in an assertion, which would ideally generate a custom message. These will also be useful for mocking when we want expectations on types.