* adds pass 3 to dequeue from channels eagerly

various other cleanup/churn:
  * use switch on cas->send in each pass to
    factor out common code.
  * longer goto labels, commented at target
  * be more agressive about can't happen:
    throw instead of print + cope.
  * use "select" instead of "selectgo" in errors
  * use printf for debug prints when possible

R=ken2, ken3
CC=golang-dev, r
https://golang.org/cl/875041