Move the parser for benchmark output from cmd/benchcmp into its own
package, benchmark/parse.
The majority of the change is just moving code around. Instead of
implementing the '-best' flag in ParseBenchSet, it is now implemented in
its own function 'selectBest' in cmd/benchcmp.
Bench.Ord (the ordinal position of a Bench within a BenchSet) has been
exported.
Change-Id: Id27032a220f9ff2596117b58b86243998695a804
Reviewed-on: https://go-review.googlesource.com/2102
Reviewed-by: Rob Pike <r@golang.org>
