Revamp interface bindings

* Extract legacy callback interface objects
  out of interface objects. Closes whatwg#78.
* Clarify that legacy callback interfaces are function objects.
* Give them a length property. Closes whatwg#279. Closes whatwg#83.
* Require new for Named Constructors. Closes whatwg#275.
* Rely on ES abstract operations for defining,
  named constructors, interface objects, and
  legacy callback interface objects.
* Disallow using the [NoInterfaceObject] on
  callback interfaces with constants.