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.
style nitpick: can you add a space after the comma in function declarations/calls? (here and below)