handle rest elements in computed properties

[Rich-Harris]

Alright, let's fix #1540 once and for all. This adds support for rest elements in computed properties. It does so in a way that avoids the props.props.props.props thing we discovered — in a case like this...

computed: {
  foo: ({ x, ...y }) => y
}

...foo will be excluded from the object passed in. This might seem slightly magical, but I think it matches people's intuitions of how this should work.

[Conduitry]

Hrm I'm not thrilled with having to do this ... Is it practical to change how whitelisted globals work so that they don't get assigned into the component's data? I actually wasn't aware or had forgotten that this was the case.

[Rich-Harris]

It's a bit annoying, yeah, but is by far the easiest way to make globals available. The alternative would be to more cleanly separate state and the root ctx object:

// on init
const base = { JSON };

// on each change
const ctx = assign({}, base, newState);
this._recompute(changed, ctx);

That might be necessary anyway at some point, to work around some other quirks relating to merging store with non-store data, and also this.get({ computed: false }) etc. But I think it's orthogonal to this issue