This trait, like its mirror in core, unifies various types that can be used to index within a bit-slice. Individual usize indices can refer to exactly one bit within a bit-slice, and R: RangeBounds<usize> ranges can refer to subslices of any length within a bit-slice.
The three operations (get, get unchecked, and index) reflect the three theories of lookup within a collection: fallible, pre-checked, and crashing on failure.
You will likely not use this trait directly; its methods all have corresponding methods on [BitSlice] that delegate to particular implementations of it.
The SliceIndex::Output type is not usable here, because bitvec cannot manifest a &mut bool reference. Work to unify referential values in the trait system is ongoing, and in the future this functionality may be approximated.
Instead, this uses two output types, Immut and Mut, that are the referential structures produced by indexing immutably or mutably, respectively. This allows the range implementations to produce &/mut BitSlice as expected, while usize produces the proxy structure.