Promise logic is broken

[nicolas-grekas]

• the return value of rejection callback should reinject to fulfill the promise and recover from the error - this is not supported
• chaining promises/callbacks is wrong, see https://blog.domenic.me/youre-missing-the-point-of-promises/
• return from a promise can be a promise too, in which case it should we unwrapped - detailed in #32 already

I'd recommend relying on guzzle/promise to fix all these issues as the implementation there is correct. See also https://promisesaplus.com/