*   Complete overhaul of *GHC.TypeLits.Witnesses* to more or less match the
    interface of singletons.  This changes the role of the module to be a
    "lightweight" version of the singletons library as it pertains to `Nat` and
    `Symbol`.
*   Removal of the *GHC.TypeLits.List* module.  We could perform the same
    overhaul as *GHC.TypeLits.Witnesses*, but it is complex enough that it
    would be better to just use *singletons* instead at that point.
*   Remove all external dependencies, except for *base* and *dependent-sum*.
*   Break compatibility with GHC < 8.2