November ES6 spec changes

This adds/removes/changes the following tests:

* RegExp.prototype methods
* Symbol.species
* String.prototype.includes
* WeakMap/WeakSet.prototype.clear
* Array.isArray on proxies
* JSON.stringify on proxies

This closes some but not all of #327.

It also includes a nested destructuring rest test (as described in #336) which I think is accurate, but maybe could do with an expert opinion.