Expose TwoWaySearcher's next and next_back functions

[sgodwincs]

Handles #10 as the minimum required to use TwoWaySearcher in a Searcher impl.

There is some ability for API misuse here since new takes a needle, but doesn't save it meaning next and next_back need to be given the same needle again to work correctly. It may be worth storing the needle in the searcher, but this would require a lifetime to be added.