New methods findprev and findlast

While working on something else, I found I wanted a `findlast` method in analogy with `findfirst`.
Since `findfirst` is implemented in terms of `findnext`, I implemented `findlast` with the same
relation to a new `findprev`.

While working, I also noticed there were no tests for `findnext`, so I added a few.
Actually, that line only copied the style from `findfirst` a few lines up. I added a commit to change them both, but the docs are in no way consistent here...