[Js.Obj]: add immutable assign function

[anmonteiro]

• Object.assign is mutable
• a common idiom is Object.assign({}, foo, bar) to create a new object

the Js.Obj module could have a function that does this. e.g.:

external assignImm :
  (_[@mel.as {json|{}|json}]) -> < .. > Js.t -> < .. > Js.t -> < .. > Js.t
  = "Object.assign"

let () = Js.log (assignImm [%mel.obj { foo = 1 }] [%mel.obj { bar = 2 }])

$ melc -ppx melppx x.ml
// Generated by Melange
'use strict';


console.log(Object.assign({}, {
          foo: 1
        }, {
          bar: 2
        }));

/*  Not a pure module */

[andreypopp]

May I suggest the name Js.Obj.merge