Improve Mux1H QoR

The previous implementation did not take advantage of the one-hot
property of the input, resulting in a priority encoder.

I also removed some redundant constructors (leveraging Vec <: Seq).
Improve PriorityEncoderOH QoR

Exposing a higher-level priority encoder structure improves QoR for
Synopsys DC, at least.  (It also results in fewer Chisel nodes.)
Remove old "otherwise" object

This appears to be a holdover from when "when" statement priority was
inverted.  Now it only serves to create bugs.