Revamp interface bindings (#313)

* Extract legacy callback interface objects
  out of interface objects. Closes #78.
* Clarify that legacy callback interfaces are function objects.
* Give them a length property. Closes #279. Closes #83.
* Require new for Named Constructors. Closes #275.
* Rely on ES abstract operations for defining
  named constructors, interface objects, and
  legacy callback interface objects.
* Disallow using [NoInterfaceObject] on
  callback interfaces with constants.
* Don't require an interface prototype object for constants.
  Callback interfaces do not have an interface prototype object.
  Their constants sit on the legacy callback interface object itself.
  Closes #281.

Closes #283.