The change adds specialized type algorithms
for slices and types of size 8/16/32/64/128.
It significantly accelerates chan and map operations
for most builtin types as well as user structs.

benchmark                   old,ns/op   new,ns/op
BenchmarkChanUncontended          226          94
(on Intel Xeon E5620, 2.4GHz, Linux 64 bit)

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4815087