io: reimplement Pipe
io: reimplement Pipe

No known bugs in the current pipe,
but this one is simpler and easier to
understand.

R=iant
CC=golang-dev
http://codereview.appspot.com/4252057

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang