Use abstract operations instead of internal methods

[domenic]

Internal methods are misleading. It is better to restrict internal to "internal slots," i.e. things we need to store directly on the object, but privately.

We should follow the ES spec and use abstract operations instead.

Note that e.g. [[onCancel]] would remain in an internal slot, since it is a function we are storing. Whereas [[callOrScheduleAdvanceQueue]] would become an abstract operation.

This is a fairly easy, mechanical, and unimportant issue. Thus I will feel guilty if I work on it before getting through the more important stuff. If anyone else wants to give it a shot, patch welcome :).

[domenic]

Done as part of the Bikeshed conversion.