Object Rest/Spread Properties

This proposal was well received when presented to TC-39 and will continue to get work for ES7. Let's start experimenting with it.

https://gist.github.com/sebmarkbage/aa849c7973cb4452c547

@jeffmo @benjamn
Can you add a non-declaration variant of this test too?
i.e. `z = {...x}` (no `let`)