Mutually recursive types

[SquidDev]

I've been meaning to file this for a year, but better late than never I guess...

While we haven't encountered a need so far, we need a way to represent mutually-recursive types[1]. I'm assuming we can just go for the normal OCaml/SML:

type odd 'z = 
| OddS : even 'n -> odd (S 'n)
and even 'z = 
| EvenZ : even Z 
| EvenS : odd 'n -> even (S 'n)

I assume, while doing this, it may also be useful to add support for mutually-recursive class instances.

[1] So technically you can represent mutually-recursive types as an inductive type indexed by a "which type is this" type, but I'd rather not.

[plt-amy]

We also need mutually recursive type functions which is going to be a bit more complicated to write..