* Merge the runtime::{io, time} modules into runtime::driver
* Create a concret Driver type which encompases the conditionally
  compiled `io` and `time` drivers
* This avoids having other code directly refer to
  `runtime::time::Driver` etc. so that other drivers can be added
  transparently

* This offers everal benefits:
 - fewer spurious wakeups: tasks blocked on a `Signal` instance will
   only get woken up when their respective signal is received rather
   than on *any* signal
 - using fewer file descriptors: we now need one file descriptor *per
   driver/runtime* rather than one per `Signal` instance for receiving
   wakeups

* This avoids lazy registration and synchronization during every turn