Don't foster-parent text nodes that consist only of whitespace.
(I implemented this entirely in inTableIM instead of creating an
inTableTextIM, because the sole purpose of inTableTextIM seems to be
to combine character tokens into a string, which our tokenizer does
already.)

Use parseImpliedToken to clarify a couple of cases.

Handle <style>, <script>, <input>, and <form>.

Ignore doctype tokens.

Pass 20 additional tests.

R=nigeltao
CC=golang-dev
https://golang.org/cl/6117048