generate: update to new stringer format

During code review for this post, dsymonds noticed a way to simplify
the generated code. That simplification is now checked in, so bring the
post up to date.

Change-Id: If29587332c8cc210ecac4843e9dd45dfa01118f7
Reviewed-on: https://go-review.googlesource.com/2020
Reviewed-by: Andrew Gerrand <adg@golang.org>