Directions methods should return some kind of Cancellable protocol instead of URLSessionDataTask

Right now all methods in Directions class return URLSessionDataTask instance directly. This makes subclassing Directions harder, as it leaks internal implementation of it (i.e. it uses URLSession underneath). As far as I noticed, returned tasks are only used for cancelling them, so it is possible to just introduce Cancellable protocol in form of:

public protocol Cancellable {
    func cancel()
}

with extension for URLSessionDataTask:

extension URLSessionDataTask: Cancellable {}

and declare all methods to return it instead of URLSessionDataTask. This will allow to make subclasses of Directions (e.g. one using other web service than Mapbox's one or some offline solution) without being tied to URLSession.

Start by locating the Directions class and inventorying its methods that return URLSessionDataTask. Review how callers use those returned tasks, then update the public return type to the proposed Cancellable abstraction and verify that cancellation still works across the affected methods.