This tweaks the spec to use the new “built-in async function” machinery in tc39/ecma262#2942. It resolves #43. See also #31, #36.

• It uses the same “This async method performs the following steps when called” language as proposal-async-iterator-helpers.
• It replaces returning Completion Records with direct returning of array values, which are now implicitly wrapped in Completion Records by the definition of built-in async function objects.
• It removes the wrapping async Abstract Closure, which is now implicitly defined by the definition of built-in async function objects.
• It tweaks the ending note to match the language in Array.from’s note.
• It lastly updates ecmarkup to v21.2.0 and ecma262-biblio to v2.1.2869.

This is the last step before creating a pull request to tc39/ecma262. We’re almost there.

@nicolo-ribaudo, @ljharb, since you reviewed it before for Stage 13 (#14), would you mind reviewing before we merge this?