map::OccupiedEntryRef::key should return &K rather than &Q

[programmerjake]

returning Q unnecessarily throws away the key type, which may have extra information needed by the user, e.g. the Rc you want to clone.

example where returning K is useful:

struct Interner<T: Eq + Hash> {
    values: HashMap<Rc<T>, ()>,
}

impl<T: Eq + Hash> Interner<T>
where
    Rc<T>: for<'a> From<&'a T>,
{
    fn intern(&mut self, v: &T) -> Rc<T> {
        match self.values.entry_ref(v) {
            EntryRef::Occupied(entry) => Rc::clone(entry.key()), // getting `&Rc<T>` instead of `&T` avoids allocating a new `Rc<T>`
            EntryRef::Vacant(entry) => Rc::clone(entry.insert_entry(()).key())
        }
    }
}

[Amanieu]

I think this is a good change and would be happy to accept a PR for it (since EntryRef is still unstable).