Replace the String method with a Format method so we can use it for extra
formats.
Add some tests to make sure it is all correct
Change-Id: I39f361ffba036fad99c93f8c0944164f7cf199ec
Reviewed-on: https://go-review.googlesource.com/c/tools/+/227486
Run-TryBot: Ian Cottrell <iancottrell@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
