I wanted to do this for a long time. The guts of Promise are pretty
complex, and it's easier to understand with the spec right next to it.
Also found a couple of issues along the way :^)
Both at the same time because many of them call construct() in call()
and I'm not keen on adding a bunch of temporary plumbing to turn
exceptions into throw completions.
Also changes the return value of construct() to Object* instead of Value
as it always needs to return an object; allowing an arbitrary Value is a
massive foot gun.
The element-resolving functions on the Promise constructor are all very
similar. To prepare for more of these functions to be implemented, break
out common parts into a base class.
