investigate how to create js class in ocaml

[bobzhang]

it still makes sense to allow create js classes in ocaml, we can do functional object oriented programming, the benefit is that methods are attached to object, think about it, if we want to
customize comparison behavior for int64, one possible way is below (tricky parts like how to handle this etc)

currently we can do this

function Int64(lo,hi){
   this.lo = lo;
   this.hi = hi; 
}
Int64.prototype.compare = ..

class type t = object
   method lo__r : nativeint 
   method hi __r : nativeint
   method compare__2 :  t -> int 
end
external create : nativeint-> nativeint -> t = "Int64" [@@js.new]
external compare :  t -> t -> int = "compare" [@@js.send]

It would be nice that we can do it all in ocaml

[bobzhang]

see #148 for the motivation.

I think we can allow a special class to be created in ocaml using a ppx:

[%%js.class fun  lo hi -> 
{ compare = ....
}
|]

[bobzhang]

proposal:

object (self)
  val field0 = x >  0
  val field1 = y < 1
  method bark = field0 , field1
  method fark = field0 , field1 + self##bark
end

(* let  v = object method x = 3 method  z =  y <-3 ; 3 end;; *)

object (self : 'self_type)
  val field0 : 'a0
  val mutable field1 : 'a1
  method bark :
    (self : 'self_type Js.t) -> (field0 : 'a0) -> (field1 : 'a1)  :
    'bark =
    field0, field1
  method fark :
    (self : 'self_type Js.t)
      (field0 : 'a0)  (field1 : 'a1) : ' fark =
    field0 , field1 + self##bark
end

Note that we only need expose method type (no val type) follow the ocaml convention, it would be

< bark : 'bark ; fark : 'fark > Js.t

about mutability, we can always use ref, or we reuse OCaml mutable syntax, but we need do a check for x <- which is more work

[bobzhang]

see #401

[bobzhang]

close this as a duplication of #401