Add method to get a map entry by index

[bkragl]

This change adds a method get_index_entry to IndexMap, which is the by-index counterpart to entry. The returned struct IndexedEntry provides the same methods as OccupiedEntry, except replace_key.

A concrete use case where I found this useful is having a map: IndexMap<_, Vec<_>> with the invariant that each Vec is non-empty. Imagine we want to pick an entry at a random index and pop a value from the Vec. Without this change we'd have to use get_mut_index and swap_remove_index, and ensure that we use the same index. With get_index_entry we can write:

let mut e = map.get_index_entry(index).unwrap();
let v = e.get_mut();
let x = v.pop().unwrap();
if v.is_empty() {
    e.swap_remove();
}

[cuviper]

This does seem like a reasonable API to add, thanks!

[cuviper]

The name is okay, but just to bikeshed a little, what do you think about get_index_entry and IndexedEntry? I think that may fit a little better with get_index and get_index_mut, and "get" methods in general returning Option, as well as similarity to the other adjective-Entry types.

[bkragl]

I dig IndexedEntry, and also renamed the method to get_index_entry. Some other options for the method would be entry_index (fits the existing entry method and get/get_index pattern, but feels odd) or indexed_entry (fits the return type). I don't have a strong opinion, but I'm happy to do another revision if somebody else has.

[cuviper]

Looks good!

[cuviper]

My last note is that this should move down near get_index because of the relaxed impl constraints there -- this does not (and will never) need K: Hash + Eq, S: BuildHasher. But I'll go ahead and move that myself, since you've allowed maintainer pushes.