Chain over the "rejected" side of an `Async`

I would like to request the ability to chain over the left or “rejected” side of an Async. This would be helpful to move values from the left side of the Async to the right.

getAsync()
  .chainRej(ifElse(isErrorOk, constant(Async.of('Error Ignored')), Async.Rejected))

I’m currently doing this with a helper function.

// chainRej :: (a -> Async e a) -> Async a b -> Async e a|b
export const chainRej = curry((fn, a) =>
  Async((rej, res) => a.fork(x => fn(x).fork(rej, res), res))
)

[mgreystone]

bichain sounds like a win to me.

[dalefrancis88]

@mgreystone There is a PR ready for bichain sitting here, https://github.com/evilsoft/crocks/pull/394 if you have time, check it out and let me know if you see any opportunities for improvement