add combinators h_skip, h_seek, h_tell

[pesco]

Packrat only for now.

Obviously, h_skip could be implemented for regular and CF backends, too. The same probably goes for h_tell because it doesn't actually match anything. It's a fancy h_action on epsilon, basically.

Finally, if one wanted to be perverse, even h_seek can probably be made to work for the other backends. After all, it's just altering the timeline in terms of what input will appear after it... Anyway, that's not what this PR is about. :)