feat: support @mel.as in variant definitions

[anmonteiro]

fixes #742

• still WIP, this just works for constant constructors
• pattern matching compilation still broken

TODO:

• pattern matching compilation
• non-constant constructors
• check option-like shape
• extensible variants? (already use MEL_EXN_ID)

[anmonteiro]

edge case to sort out:

type (_, _) x =
  | [] : ('a, 'a) x [@mel.as "lol"]
  | ( :: ) : 'a * ('ty, 'v) x -> ('a -> 'ty, 'v) x

let x : _ x = []

let rec f : type a b. (a, string ref) x -> string =
 fun x -> match x with [] -> "empty" | _ :: xs -> f xs

generates:

function f(_x) {
  while(true) {
    const x = _x;
    if (!x) {
      return "empty";
    }
    _x = x.tl;
    continue ;
  };
}

const x = /* [] */"lol";

[dmmulroy]

LFG