Add experimental retained-compose artefact

[marcellogalhardo]

First interation for built-in compose support. Usage looks like the following:

 @Composable
 fun MyComposable() {
     val vm = retain { ViewModel() }
 }
 
class ViewModel(val name: String = "")

The new Lifecycle 2.5.0 also allows us to improve the original design for retained-core (public API won't change), but those improvements will be handled in a separate PR.

Fixes #62