Should `timer::CountDown` be a supertrait for `timer::Periodic`?

[hannobraun]

#67 adds a new timer trait that has timer::CountDown as a supertrait. Should the same be done for timer::Periodic?

I think yes.

cc @therealprof @jonas-schievink

[Dirbaio]

I'm going to close this, now that the old timer traits are removed #324.

Discussion for design for the new traits can go in #359 . Linking it so this issue appears there.