Spec async iterable `from()` conversion

[domfarolino]

All of the async iterable conversion was specified in #160, but it got a little complicated, so I decided to rip the async iterable conversion prose out of it, land that PR with everything but async iterable conversion support, and file this issue as a tracker issue to follow-up on that task. Mostly it just involves adding back the prose that was removed in 57b2d19, and ensure we resolve the following comments from that PR:

• Spec from() static converter #160 (comment)
• Spec from() static converter #160 (comment)