`Indexable` and `Lookup` for Generic Vectors.

Would it be possible to add instances for indexing into a Generic vector so that these type classes can be used with Unboxed or Storable vectors? Similarly would it be possible to do the same with other kinds of arrays?

I realise `keys` is based around Haskell functors and these things are not functors in that sense (they are, I suppose, functors on mythical categories of Unboxed types, though I wouldn't wish to be quoted on that).

No files or tests are named. Begin by inspecting the existing Indexable and Lookup instances in the repository and how Generic, Unboxed, and Storable vectors are represented; determine which requested array types are in scope. Done means the relevant vector instances compile and allow both type classes to be used with the requested vector variants.

Written by the indexing model from the issue text.