vector: add benchmarks.
vector: add benchmarks.

Change-Id: I4946ccc3a915c7e9365cd5fe860da673a2270fbb
Reviewed-on: https://go-review.googlesource.com/29631
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

golang/image