Creates a Subscription which receives promises, buffers them and then passes their resolved values downstream.
If the optional fail handler is provided, it'll be called with the error of each failed promise. If none is provided, the sub's ISubscriber.error handler is called, which then stops the sub from receiving further values.
fromIterable([1, 2, 3], 100)
// result 1// result 2// result 3// result done