Rework header
Added an "instances of iterables" subtest to for-of.
Added extra yield * tests.

Also:
 * Extended the 'instances of iterables' tests to the other iterator-consuming syntaxes.
 * Abstracted the test code for creating a generic iterable into a __createIterableObject function, which is included in all ES6 skeleton files and the Node test script. This is something of a readability trade-off: sacrificing transparency for clarity and brevity.
Updated yield * results for various browsers.

Also:
 * Fixed a bug which caused the for-loop strict tests to crash in Chrome 38.
 * Fixed some mistakes in the Set results for Firefox.