atdd: recursive variants and records

[elrandar]

• Add support for recursive variants through the following syntax :

  
  type record_that_uses_recursive_variant = 
  {
  v: recursive_variant;
  i: int;
  }

type recursive_variant = [ | Int of int | Record of record_that_uses_recursive_variant ]

- Add support for recursive records :
```ocaml
type recursive_class <dlang shape="recursive"> = {
  id: int;
  flag: bool;
  children: recursive_class nullable;
}

• Add option to use enum as data type for variant without data

  type planet <dlang shape="enum"> = [
  | Mercury
  | Venus
  | Earth
  | Mars
  | Saturn
  | Jupiter
  | Neptune
  | Uranus
  ]

  Closes https://github.com/ahrefs/atd/issues/392

[elrandar]

After using it for internal projects, I am not happy with the generated code in some cases, so would like to leave it open until I find the time to have an implementation I'm happy with

[mjambon]

Marking this PR as "draft" to avoid accidental merging.