Consider extracting Compose support for multiplatform

People are already starting to play with running Compose on non-android platforms ([eg](https://twitter.com/martinbonnin/status/1297278514085978113?s=19)), 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.