Reason: suggestions: make rec type explicit

[bobzhang]

non recursive types are simple ones, simple is better, also consistent with let rec

so that

type t = int list 
type rec t = Nil | Cons of t * t list 

[tekknolagi]

Why add this is OCaml does not require it?

[jordwalke]

@tekknolagi OCaml has rec by default for types and you have to specify nonrec to make it non-recursive. It's kind of confusing because it's different convention than value let bindings which have nonrec by default and make you specify rec to achieve recursive bindings.
I would rather them be consistent, and I don't care which (either both types and values recursive by default or not as long as they match).

[chenglou]

Merging into #394. Let's do it!