Deduplicate thenable coercion, and bring back thenable coercions weak map

[domenic]

The code of ThenableToPromise and Promise Reaction Functions both does a form of thenable coercion:

• The former tries to coerce thenables straight-up into promises, so that its caller can use a trusted promise then or just pass the value on through.
• The latter tries to send a "then" message to the return value of [[Handler]] using deferred.[[Resolve]] and deferred.[[Reject]] as the parameters, or just call deferred.[[Resolve]] if it's not a thenable.

These are pretty similar, but I believe I'm making them unnecessarily distinct in my current set up. E.g., if I moved some of the caller behavior into the former, the parallel would be clearer.

Once these functions are deduplicated, I will probably have an easier time reintroducing the thenable coercions weak map. I gave it a stab in bring-back-thenable-coercions branch but it did not pass the existing tests because I only did it inside ThenableToPromise and not inside Promise Reaction Functions.

I am reasonably happy with the verbiage surrounding the weak map in that branch, so the work here subsumes #39. Also I can try to be sure not to reintroduce any reentrance bugs, so let's subsume #61 in here too.