Well, three different ways to name the "start from the back" variant seemed bad to me, and a `r` prefix didn't seem very self documenting. 

From the renamings I did here, only `rfind` is used at all in a few places.

Also, I have the feeling `find` in general could be turned into a double ended iterator, which might mean we could get rid of a `rfind` function completely - see #9391.