fix warning and upgrade version

add assert for backwards compatibility

Make Reactive more typesafe

Rename State to Reactive to avoid namespace issues,

remove old Reactive code since it duplicates
ChangeNotifier behavior

forgot to export state.dart

Added State class

Remove unnecessary files, update version

Added Reactive mixin class, update to 2.1.0-dev.1

Implement pub publish suggestions for versioning as a dev.1 opt in ve…

…rsion

updated version