Add Canceller for removing progress/then handlers.

[inamiy]

(JavaScript) Promise is normally designed to only attach then-handlers to preceding task, but it often lacks in those removability, especially when user wants to remove them one-by-one.
(Promise usually removeAll() when it is deallocated)

This kind of casual & rough monad structure is not an issue when only dealing with a single future value, but when multiple progressing values come in, without a support of one-by-one invalidation, it will degrade efficiency of multiple task collaboration, i.e. ReactKit.

This pull request will add one-by-one invalidation of progress/then handlers by using Canceller.
Unlike publish-subscribe-model which normally returns disposable token as returning value,
I decided to use inout parameter to maintain Promise pipelining.

Usage

let task = ...
var canceller: Canceller? = nil

task.progress(&canceller) { ... }.then { ... }

// at some point before `task` completes
canceller?.cancel()   // removes progressHandler

[inamiy]