Rework duck-typed parent into SlicedParentNode class

[botandrose]

The main difference here is that childNodes is now a live calculation, based on a bounding box defined by the originally-passed node's siblings. In other words, a parent node containing a live slice of its children.

Also, the SlicedParentNode name is better and more intention-revealing, I think.

Lastly, it is now a JavaScript class, which definitely goes against the overall C-ish style of Idiomorph. But since we're emulating a concrete aspect of the OOP DOM API, I think it makes sense.

Fixes #124, and the return value issue in #126.

[botandrose]

npm run perf v0.7.2 shows no detectable perf difference from v0.7.2 on any benchmark, which is what I think we should expect.

Its slower to recalculate the childNodes each call, as opposed to returning a static array, of course. But that recalculation should only happen a few times per morph when we're morphing outerHTML style, and thus be negligible.

All benchmarks ARE using the outerHTML morph style, though, so this code is definitely getting run!