Prevents spinning in "Activations.run" when a thread is blocked,
then awakened, and added to the sequence already being traversed.
By adding the awakened event to the front, we don't traverse it
again in the same iteration.

Signed-off-by: James Bielman <jamesjb@galois.com>
Acked-by: Anil Madhavapeddy <anil@recoil.org>
Merge branch 'master' of github.com:mirage/mirage-platform