Summary of changes:
   - Removed RankNTypes; added Rank2Types, UndecidableInstances
   - Revamped Command interface to be simpler and more monadic
   - Refactored key/command processing into new module RunCommand
   - Begin infrastructure for emacs/vi modes to have their own state.
   - Make Monad boilerplate a little simpler
   - TODO: The Windows build is broken.