I [recently realized that `.last()` might not call `next_back()` when it
is
available](https://qsantos.fr/2025/01/01/rust-gotcha-last-on-doubleendediterator/).
Although the implementor could make sure to implement `last()` to do so,
this is not what will happen by default. As a result, I think it is
useful to add a lint to promote using `.next_back()` over `.last()` on
`DoubleEndedIterator`.
If this is merged, we might want to close #1822.
changelog: [`double_ended_iterator_last`]: Add lint for calling
`Iterator::last()` on `DoubleEndedIterator`
631d9a2c5c