Add new functions setNodeSeqNode, appendNodeSeq, appendNodeSeqNode.

Passes toolstash -cmp.

Change-Id: I6c1745b1108dea45a2c0d029b9de1917ae17a962
Reviewed-on: https://go-review.googlesource.com/20196
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>