Add a function to get the key from a HashMap

[iqualfragile]

Basically just

pub fn get_both<Q: ?Sized>(&self, k: &Q) -> Option<(&K,&V)>
        where K: Borrow<Q>,
              Q: Hash + Eq
    {
        self.search(k).into_occupied_bucket().map(|bucket| bucket.into_refs())
}

sometimes you need a reference to a key with the lifetime of the collection.

[oli-obk]

If OccupiedEntry::key returned the reference with the proper lifetime, we could already do this in usercode.

[iqualfragile]

@oli-obk the entry api would not work, because it requires &mut, this usecase would require the ability to have multiple references and therefore just &

[dtolnay]

I don't love the name get_both but I agree that this is basic functionality that HashMap should expose. I would welcome a PR and we can refine the name separately.