Support computed state

[Chandler Prall]

Use case

1. sharing computed state - the Current Episode in a playlist is computed from a list of episodes and a timestamp, and is a useful thing to compute once and share
2. reacting to the proper state change - A media element emits the current playhead at least 4 times a second. Reacting to this in the listener can result in repeated expensive computations. Instead, a Current Episode  derived/computed state could be listened to directly, limiting reactive computation only to times when the value has changed.

[Chandler Prall]

This will need to maintain a dependency tree between computed values to avoid creating infinite update loops.

[Chandler Prall]

#10 is PR for computed state