For now this applies to Haskell only, and it may turn out to be tricky for the Rust implementation.
In practice, the limitation hasn't turned out to be important, and we could even go the other way and use `Integer` everywhere. This does however at least help with debugging, as well as just being conceptually right.
The `nil` and `(/\)` functions are intended to be overloaded to work for other list-like things in a later commit, and from there we will investigate using `OverloadedLists` and `RebindableSyntax` to recover standard list syntax, although there are probably limitations due to `(:)` being special.
Simplifies encoding code slightly, and potentially saves a lot of time for failing tests. Plus we've always been using `T.show` in practice anyway, so it's an easy change to make.
