Testable implementation is starting to lag

[domenic]

As of 19318d3 it is not up to date with the spec, as it does not handle exceptions in Promise.race per the new behavior. Also, as discovered in #42, internal properties are not inherited, so the way they are currently done is wrong. And the @@create stuff was never quite right.

As #6 is implemented, the delta between how the spec talks about things (with completion values) and how the testable implementation does (by not delving to that level) will grow. So I'll take a pause on updating the testable implementation alongside the spec. Once things settle, especially #6, I will go back and re-do it.

• @@create vs. constructor semantics are too fake
• Internal properties should not be inherited; currently scheme is too simple.
• Completion values are not handled
• Make sure all the tests still pass with the updated implementation