The bytes package iterators return subslices, not substrings.

Updates #61901.

Change-Id: Ida91d3e33a0f178edfe9a267861adf4f13f9a965
Reviewed-on: https://go-review.googlesource.com/c/go/+/647875


Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
(cherry picked from commit ff27d270)
Reviewed-on: https://go-review.googlesource.com/c/go/+/648095


Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Bypass: Cherry Mui <cherryyz@google.com>