Add ObservableVector::swap_remove

[bnjbvr]

This would have been handy for matrix-org/matrix-rust-sdk#4346 (review). I suppose that to make the proposed fix work, we would need swap_remove() to return the index of the swapped value too (even though one could imagine it's always the index of the last element).

[jplatte]

I'm not really convinced, tbh. That PR didn't end up with something like swap_remove, the only case I've seen it used is where ordering is irrelevant, because it's faster for Vec and similar collection types.

I suppose for Vector, if it consists of a single chunk the same thing applies. But for larger instances, I think that's not the case. Also, having an extra VectorDiff variant also means that every consumer has to handle that as well. I've thought of ways around that, but I don't think there's a great solution.