Add support for double ended days+weeks iterators

[teobouvard]

This allows users to use .rev() on .iter_days() and .iter_weeks(),
in order to iterate on dates backwards.

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?

[djc]

Thanks!