Computationally universal register machine implementation at the type-level.
This formulation is identical to the [Lambek
machine](https:en.wikipedia.orgwikiCounter_machine), with the addition
of an explicit Halt instruction, for convenience.
This means (or rather, this is made possible by the fact) that that
Haskell's type system is Turing complete (at least with
TypeFamilies and UndecidableInstances).