Consider making a type-stable version of IndexLens #127
Description
PropertyLens is 'type stable' (loose wording here) in that the symbol associated with it is stored as a type parameter. IndexLens isn't, because the actual indices are only values, not types. I'm somewhat curious as to whether there is any benefit to making a type-stable version of it. That might, for example, allow us to have more guarantees on type stability.
Note that along with making a normalised version of ComposedFunction (sort of done in #123) this essentially means making our own version of Accessors. I don't think this is a terrible idea because the scope is limited to IndexLens, PropertyLens, and compositions thereof.