String.rev

For the life of me I could not find this function in the source and now I'm pretty certain that it's missing.

This might be a good time to discuss this naming convention in batteries. What's the deal with the `rev_in_place` vs `mirror_in_place` stuff? I much prefer `rev_in_place` but of course we can't remove `mirror_in_place`. Nevertheless would it be good to have both in BatString for consistency?