benchmark/parser, cmd/benchcmp: improve names of exported identifiers

Most of the names have been brought in line with the names used in
testing/BenchmarkResult. For example, NsOp becomes NsPerOp.

Additionally, "Bench" becomes "Benchmark" and "BenchSet" becomes "Set".

Change-Id: I7dfca68a804e285a87ab9692b5bb99ccb676da7f
Reviewed-on: https://go-review.googlesource.com/2610
Reviewed-by: Rob Pike <[email protected]>

• benchmark/parse/parse.go[diff]
• benchmark/parse/parse_test.go[diff]
• cmd/benchcmp/benchcmp.go[diff]
• cmd/benchcmp/benchcmp_test.go[diff]
• cmd/benchcmp/compare.go[diff]
• cmd/benchcmp/compare_test.go[diff]