Add `View` version of `IndexMap` and `IndexSet`

IndexSet depends on the View type for IndexMap being available.

IndexMap is currently implemented with a CoreMaptype that looks like:

struct CoreMap<K, V, const N: usize> {
    entries: Vec<Bucket<K, V>, N>,
    indices: [Option<Pos>; N],
}

It is not possible to construct a View for this type like for Vec since the N would have to be erased twice.

We could solve that issue by "inlining" the indices into the entries:

struct CoreMap<K, V, const N: usize> {
    entries_indices: [(MaybeUninit<Bucket<K, V>>, Option<Pos>); N],
    len: usize,
}

Then only one N needs to be erased, and this can be implemented. However this requires changing the implementation significantly.

It might even be possible to get rid of the len field (I think an entry is initialised if and only if a index points to it, and only one index can point to it at a given time?).

Start by reading the existing CoreMap, IndexMap, IndexSet, and View implementations to understand how the const generic N is represented and erased. Compare the proposed separate-index and inlined-index layouts, then determine the required design before implementing View support for IndexMap and IndexSet.