Add downcount

Add a down counter named downcount
Add adder and regn

Add an adder named adder and register named regn. Rename parameters.
Add mux_2to1

Add a 2-to-1 multiplexer named mux_2to1
Add averager and averager_tb

Add averager and averager_tb which exhibit basic functionality; ES has
been changed to a Mealey output in averager