exp/html: change a node's children from a slice to a linked list.

exp/html: change a node's children from a slice to a linked list.

Also rename Node.{Add,Remove} to Node.{AppendChild,RemoveChild} to
be consistent with the DOM.

benchmark                      old ns/op    new ns/op    delta
BenchmarkParser                  4042040      3749618   -7.23%

benchmark                       old MB/s     new MB/s  speedup
BenchmarkParser                    19.34        20.85    1.08x

BenchmarkParser mallocs per iteration is also:
10495 before / 7992 after

R=andybalholm, r, adg
CC=golang-dev
https://golang.org/cl/6495061