CharToNat and NatToChar type families

@RyanGlScott’s work on singletons revealed that built-in type families added in 7f3524ef are insufficient to implement a promoted variant of Show Char.

For Show, we need CharToNat. And with NatToChar to accompany it, we can also define promoted Enum Char.

These type families were originally part of !3598, but it is currently on hold because it’s unclear whether we want to add so many built-in definitions (and also because the author currently has no time to rebase it and clean it up, at least in time for the %9.2.1 release). However, now there’s a clear case for CharToNat and NatToChar, so this ticket is to track their addition.

Patch here: !5258 (closed)