go/format: add simple benchmark framework and basic benchmark

go/format: add simple benchmark framework and basic benchmark

For now, this CL adds as a first benchmark the formatting of
a 10,000 element array literal. It is easy to add additional
test cases as we see fit.

	name                   time/op
	Format/array1-10000-4    26.7ms ± 7%

	name                   speed
	Format/array1-10000-4  2.43MB/s ± 6%

	name                   alloc/op
	Format/array1-10000-4    5.52MB ± 0%

	name                   allocs/op
	Format/array1-10000-4      119k ± 0%

Updates #26528.

Change-Id: Ic8ec8f70160d122b877740412d4d4406f5f4b345
Reviewed-on: https://go-review.googlesource.com/c/153642
Reviewed-by: Ian Lance Taylor <iant@golang.org>