bytes: Benchmark Buffer's compactions
bytes: Benchmark Buffer's compactions

This benchmark verifies that CL #8173043 reduces time spent
sliding the Buffer's contents.

Results without and with CL #8173043 applied:
benchmark                        old ns/op    new ns/op    delta
BenchmarkBufferFullSmallReads       755336       175054  -76.82%

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/8174043

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

mhennings/marcohennings-go,d0f/go-zh,mhennings/marcohennings-go,d0f/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,webfd/go-zh,mhennings/marcohennings-go,scirelli/scirelli-go,bryanxu/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,mhennings/marcohennings-go,bryanxu/go-zh,mhennings/marcohennings-go,d0f/go-zh,webfd/go-zh,scirelli/scirelli-go,d0f/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,scirelli/scirelli-go,rdp/rogerpack2005-golang,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,mhennings/marcohennings-go,mhennings/marcohennings-go,rdp/rogerpack2005-golang,scirelli/scirelli-go,bryanxu/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,d0f/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,webfd/go-zh,sanjosh/sanjos100-tipc