Current results on 2 core darwin/amd64:
BenchmarkGoroutineChain		351 ns/op
BenchmarkGoroutineChain-2	3840 ns/op
BenchmarkGoroutineChain-4	4040 ns/op
BenchmarkConcGoroutineChain	350 ns/op
BenchmarkConcGoroutineChain-2	875 ns/op
BenchmarkConcGoroutineChain-4	2027 ns/op

R=r, rsc
CC=golang-dev
https://golang.org/cl/6332054