Implement chain

[vittoriom]

The API should take an array and a closure that takes an element and returns a Future, and return a Future that will execute all the single futures one by one (sequentially)

[vittoriom]

From mproberts/objc-promise:

Calls each supplied block with the result of the promise from the previously executed
block. If any promise rejects, the chain is broken. If all promises resolve, the result
of the last promise will be returned.

returns a promise which is resolved with the result of the last executed block