Pairs has peek() but does not implement Peekable and has a different interface

[frp]

$SUBJ. This means that next_if and other things I came to expect from the iterators that have peek() are not available.

Also, Pairs::peek() returns Option<Pair<...>> instead of Option<&Pair<...>> like the standard library's Peekable::peek() does.

A concern with just fixing it is that it's a breaking API change. But maybe it would make sense to do this if you're planning a new major release at some point?