API changes for next observe version

[jmesserly]

proposal for v0.20.0:

• use sourcegen to generate getter/setter expansion
• split observable to be only the interface (consider splitting Observable into interface vs mixin #10)
• optimize subscription mechanism
• add getter support

New APIs inspired by: https://github.com/sigmundch/DEP-member-interceptors/tree/master/example/observe

remove older features:

• PathObserver/Bindable
• ChangeRecord marker interface (at the moment, it's only there to support PathObserver)
• dirty checking

[sigmundch]

Here is an updated prototype of what I have in mind: https://github.com/sigmundch/observe_gen

In particular, like the proposal the prototype:

• is designed to support sourcegen
• optimizes the subscription mechanism (streams are not added by default)
• supports getters, and hence does not need path-observer
• removes dirty-checking, PathObserver, etc

One differences with your proposal above is that instead of spiting the observable interface, we completely removed it. We change the granularity of observability to the point where it may not be needed.

There are still a couple patterns I have not yet looked into:

• support for list and maps
• the notion of a change record