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>

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