Abstract out shareable parts of the summarizer spec

These can all power other AI specs.

Notable changes that aren't just moving stuff around:

* The creation of the AIDestroyable mixin to help centralize destruction logic.
* The use of dependent abort signals to make it easier to handle both destroying a model object and aborting an operation.
* We now acknowledge that starting a download might fail, and handle that.
* Added missing awaits to the availability language example.
* Added missing task queuing to availability(). Fixes #39.
* Fixed a bad link to summarizeStreaming() in the errors section.

Author: domenic