model `this` semantics in the FFI

[bobzhang]

taken from #174

TODO list:

• callback, event handler
• Js object without this semantics
• Js object with this semantics
• Js class

[bobzhang]

now we can model callback(event handler) like this

fun [@bs.this] obj x y -> obj##bark (x,y)

as

function(x,y){
  var obj = this;
  this.bark(x,y)
}

Next step allow to create object with this semantics

[bobzhang]

simple cases first:

object (self)
  val x = 3 
  method say x = self##x + x 
end [@bs]

would have type

object 
   val x : int 
   method say : int -> int
end [@bs]

[bobzhang]

@chenglou will this be useful to your bindings

[bobzhang]

let's leave creating JS class post-1.0

[bobzhang]

closed in favor of #2479