Implement clear() and skip_one() functionality (continuation of #287)

[ivakam]

This PR is my continuation of #287 where I've firstly rebased against the lastest master (as of writing) as well as reworked the clear() function to act more as described here.

The code passes all tests, but I have not written any new tests. As for the implementation, I think it looks fine. There's no major deviation from the existing architecture, but all feedback is of course welcome.