Provide ImplicitClock and ImplicitReset (#3714) (#3769)

Module's logic that creates the implicit clock and implicit reset within
the Module's body is now implemented in new traits that users are free
to mix in to RawModule. There are also now virtual methods implicitClock
and implicitReset that the user can override to change what clock or
reset is used.

(cherry picked from commit abc96e20

)

Co-authored-by: Jack Koenig <koenig@sifive.com>