Useless `peekable` iterator

[mcarton]

Just noticed that when refactoring. We could warn when making an iterator peekable, but never actually peeking it(/returning it/passing it to a function).

[birkenfeld]

Sounds pretty specific. Are you sure it's worth it?

[Serial-ATA]

@rustbot claim

[lukaslueg]

@birkenfeld FWIW I've seen this in the wild, too