The new flutter.mojo is ready for use.
Also cleaned up the flx code a bit. Replaced custom KeyPair class with cipher's AsymmetricKeyPair.
This changes flutter's updater package to depend on and use the new flx package. I also did a little cleanup of the Bundle interface.
This will be shared by sky/updater and flutter_tools. Currently bundle.dart and signing.dart are unmodified copies of the files from updater and flutter_tools, respectively. This is so future changes are easier to see.
