Since we're using `BTreeSet` for element constraint sets now, we might
as well use it for the selection set too. This removes the `rustc-hash`
dependency.
This tells Sycamore that the outline view of an element should update
when the element's constraint set has changed. To make the constraint
set hashable, so we can include it in the diff key, we store it as a
`BTreeSet` instead of an `FxHashSet`.
