Build `callback` object in `_async`.
Build `callback` object in `_async`.

Do most of the `callback` object construction in the `_async` function
eliminating duplicated code.

File                             Raw         Min         Zip     Min/Zip
index.js                 10337/10.09  3293/03.22  3019/02.95  1293/01.26

bigeasy/cadence,bigeasy/cadence