add missing rkyv derives

[dovahcrow]

Thanks for contributing to chrono!

• Have you added yourself and the change to the changelog? (Don't worry
  about adding the PR number)
• If this pull request fixes a bug, does it add a test that verifies that
  we can't reintroduce it?

This PR adds missing rkyv implementation (from this commit 0ec64a9) for some types.

[djc]

Thanks!