This is intended to replace the awk-based misc/benchcmp.
It mostly matches the existing misc/benchcmp.
Notable changes:
* Written in Go.
* Minor whitespace changes in the output; the tabular nature of the
output is preserved, as is most number formatting and verbiage.
* Comparisons within each section are sorted from highest change to lowest.
* Proper handling of multiple benchmarks with the same name (issue 7016).
* Does not omit benchmark comparisons for which the new value is zero.
* Adds -changed flag to only show benchmarks whose value have changed.
Useful for memory-oriented, large-scale benchmark comparisons.
* Has tests.
* Formats small ns measurements with extra precision.
Updates golang/go#7016.
LGTM=r
R=golang-codereviews, dave, dvyukov, oleku.konko, bradfitz, gobot, r
CC=golang-codereviews
https://golang.org/cl/47980043
