Analogous to the Nodes type used as a more space efficient []*Node
representation.

Passes toolstash -cmp.

Change-Id: I8341e45304777d6e4200bd36dadc935b07ccf3ff
Reviewed-on: https://go-review.googlesource.com/20793Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>