[feature] Add sugar for immutably updating js objects

"Immutably updating" a js object (creating a new copy w/ some fields changed) is currently quite cumbersome and not terribly type safe.

let someObj = {"a": "1", "b": "2", "c": "3", "d": "4"};
let newObj: Js.t {a: string, b: string} = Js.Obj.assign (Js.Obj.empty ()) someObj;
/**         ^ - you have to constrain this, because Js.Obj.assign returns type `Js.t {..}` **/
newObj##a = "awesome";
newObj##d = "things";

Proposed syntax (mirrors record update):

let someObj = {"a": "1", "b": "2", "c": "3", "d": "4"};
let newObj = {...someObj, "a": "awesome", "d": "things"};

This would desugar to

let newObj = {
  let tmp = Js.Obj.clone someObj;
  tmp##a = "awesome";
  tmp##d = "things";
}

This also requires a new function, clone, in the bucklescript std library. It couple implemented as:

let clone: Js.t 'a => Js.t 'a = fun obj => Js.Obj.assign (Js.Obj.empty()) obj;

Possible objections:

• "We shouldn't encourage this, using objects is slower than records!" 🤷‍♀️ ppl are gonna have to learn the pros/cons between objects and records sometime. Objects are super useful for js interop, and in many cases doing a clone+update of a js object that you already have will be faster than converting to a record and then updating it...

cc @chenglou @bobzhang

Start from the proposed object-update syntax and its desugaring in the issue, then locate the parser and syntax translation code in the Reason compiler. Also inspect the BuckleScript standard library area where Js.Obj.assign is defined to assess the proposed clone function. Done means the syntax and clone support are implemented with suitable coverage, but no files or tests are named in the issue.