firstChild/lastChild/nextSibling/previousSibling stepped straight to the
next sibling whenever a node was not accepted, so a FILTER_SKIP node hid
its children instead of exposing them:
<div id=root><section><a></a></section></div>, SECTION -> FILTER_SKIP
w.firstChild() // null (Chrome: <a>)
They also used the boolean `_accept`, so FILTER_REJECT (which must not
descend) and FILTER_SKIP (which must) were indistinguishable — the same
root cause #461 fixed for nextNode().
Replaced all four with the spec's "traverse children" and "traverse
siblings" algorithms, parameterised by direction so forward and backward
share one implementation and cannot drift: `edge` is first/last child and
`step` is next/previous sibling. Both consult the three-valued `_filter`
from #461, so SKIP descends, REJECT prunes, and the climb back out stops
at root or the node the walk started from.
parentNode() is left alone here: its deviations are different (it checks
only the immediate parent, and excludes root, which the spec permits it
to return), so it belongs in its own change.
Andrey Mnatsakanov
49d4b91348