Improve implicit progress handling in Task.map

[zwaldowski]

Lower-priority follow-up to #260, #265.

Ideally, progress objects created during a call to Task.map should participate in implicit progress composition: a map should take up 1 work unit by default, but 100 if a Progress is created using Progress.current() as a parent. This would have the benefit of working with Data reading and writing, Core Data fetches, as well as custom uses of implicit progress reporting.

I had a prototype like:

class Step: Progress {
    override func addChild(_ child: Progress, withPendingUnitCount pendingUnitCount: Int64) {
        setUserInfoObject(true, forKey: TaskChain.didAddChild)
        super.addChild(child, withPendingUnitCount: pendingUnitCount)
}

Then using didAddChild to perform the 1 or 100 check. But this doesn't work, because the work in the map is done by the time the map handler returns. Maybe this is as simple as moving the override to Root and making it do something immediately. I didn't investigate it further.