Consider extracting Compose support for multiplatform

[zach-klippenstein]

People are already starting to play with running Compose on non-android platforms (eg), and this library might be useful there too.

To support this, the non-android parts of this library should be extracted into a core module (ie ViewFilter, ViewStateRenderer, etc), and the compose support should be extracted as well. A good entry point for the Compose module might be either Composer or SlotTable (the latter exists now, but might go away (#58)).

I believe this would require not require a major version bump as long as things are just moving around and the Android module declared an API dependency on the core module.