IndexMap/IndexSet: deprecate remove() in favour of swap_remove #679

.remove does not convey the current behaviour that it changes the insertion order of the values in the map/set. indexmap deprecated the method in favour of the more explicit swap_remove.

We should add swap_remove where it's missing and mark remove as deprecated, following the IndexMap API.

Since I anticipate there could be discussion on whether this is the best path forward, I did not include this change in #673.