Compile time is about the same.  Getting rid of the nodeSeq interfaces,
particularly nodeSeqIterate, should produce some improvements.

Passes toolstash -cmp.

Update #14473.

Change-Id: I678abafdd9129c6cccb0ec980511932eaed496a0
Reviewed-on: https://go-review.googlesource.com/20343Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>