Opt-in mutable access on IndexSet, release 2.2.6

[cuviper]

This adds set::MutableValues as an opt-in trait for accessing IndexSet values by mutable reference, akin to map::MutableKeys. The implementation simply forwards to the map implementation while discarding the () inner "value".

Unlike IndexMap, IndexSet doesn't have regular get_full_mut or get_index_mut methods, so the "2" suffix here isn't really necessary, but I thought it might be better to stay consistent with MutableKeys on that.

Closes #322

[cuviper]

@Morganamilo would this work for you?

[Morganamilo]

This is what I was looking for thanks.