Persistent handles and event handlers

[dherman]

The EventHandler API was a step towards enabling general communication from arbitrary Rust threads back to a JavaScript thread, but it had some safety issues:

• Race conditions and memory leaks in the cross-thread memory management protocol (Race in closing EventHandler causes abort #551)
• Lack of support for managing exceptions in the event handler

We have been having a great discussion in a pre-RFC thread about an alternative design that is safer and more general. @kjvalencik is working on writing up an RFC.

[dherman]

Implemented in #622.