owners: add owners for io and strings packages
owners: add owners for io and strings packages

Change-Id: I388a2404eeb14591c86452c4c64e401691ee384e
Reviewed-on: https://go-review.googlesource.com/c/build/+/188377
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

golang/build,golang/build,golang/build,golang/build