A few post-merge fixups.

- Add changelog entry.
- Don't double-include `Composable` (it's included in `BaseMatcher`)
- Improve composable spec so that it tries to match against a value
  that does not respond to `between?`. (Nothing failed when I removed
  the `comparable?` check from `matches?`).
- Spec the failure message for non-comparables.
- Don't put `actual.inspect` in the failure message twice. Sometimes
  `inspect` output can be very large, and we usually just put it
  in the failure message once.
- Leverage more of BaseMatcher (e.g. for `failure_message_when_negated`).