Lint using `next_back` when `last` is more appropriate

[clarfonthey]

It makes sense to lint the use of next_back in cases where the iterator is not used beyond the call to next_back, suggesting to replace last instead. Additionally, implementations of Iterator::last that only delegate to next_back should be suggested to be removed.

This will likely need to be special cased to only do this for iterators which are known to optimise last into next_back through some sort of whitelist.

[sugar700]

I think I would rather go the other way around, warning about last when iterator implements DoubleEndedIterator. The problem with last is that it doesn't require DoubleEndedIterator to be implemented which means that in general case it's going to be slow, and while this can be dealt with manual implementation of that method, most iterators in standard library don't bother to do so. For those that do bother, I don't expect any particular difference between next_back and last even when the iterator is to be immediately dropped.