Benchmarks broken off from https://golang.org/cl/24723 and modified to
allocate less in the places we're not trying to measure.

Updates #16791

Change-Id: I508e4cfeac60322d56f1d71ff1912f6a6f183a63
Reviewed-on: https://go-review.googlesource.com/30357
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>