Bring back OptionalRootable trait

[frewsxcv]

From this thread on Reviewable

@Ms2ger: .map(|s| s.root()) should be just .root()
@frewsxcv: OptionalRootable (which presumably made this method on Option possible) was removed in #6150
@Ms2ger: Bah. File an issue to bring it back.

[frewsxcv]

/cc @michaelwu since they might have more context as to why the trait was removed in the SM-up

[michaelwu]

Needing to call root() at all is a bit unsafe. What you really want to do here is call get_rooted() on next_sibling/first_child. See #6684 . OptionalRootable just papers over the brokenness of dealing with raw JS<T>.

[frewsxcv]

Is this still desired?

cc @Ms2ger

[Ms2ger]

Meh, I guess.