Expands the `<content>` element to remember logical DOM

[jmesserly]

It's now capable of rerendering if logical DOM changes.

Notes:

• lightChildren, if present, indicates the logical child views. lightParent will be set for those elements.
• lightChildren is an array, because that seemed like the "simplest thing that could possibly work". We could expose linked list APIs instead (firstChild, nextSibling, etc) if we wanted.
• addLightChild/removeLightChild are a convenience, but not recommended for adding large number of items because they automatically call distributeContent(). Another option would be to schedule it and do it lazily.

[jmesserly]

oops, apparently using Custom Element constructors in a test doesn't work on Safari (and maybe IE 10, but works 11?). I'll just go back to document.createElement.

[jmesserly]

hmmm, I'm also seeing a failure in ready's unit test, maybe IE10 is expected to fail in the 0.8-preview branch? fwiw, IE11 passes and so does Safari when I run locally (on the bot it seems to have issues)

[sorvell]

LGTM. Great work John. I have some minor comments but I'm going to merge this and then we can iterate. I may tweak some small style stuff and change a couple method names.

[jmesserly]

Thank you Steve!

Yes definitely feel free with any style tweaks/refactoring. The code has come a long way since extracting in from SD, but there's probably still more to do. After a long time of restructuring the code, I'm probably blind to some of the oddities at this point :). Will be looking forward to seeing it evolve even more cleaner/nicer.