Notify changes synchronously if possible

[helios175]

We don't want to use the mainScope.launch { calculateNotifications ; then update } path if the updates are synchronous (for instance, when only extra item is updated, or when going from empty to not-empty).

This PR changes:

• Update.generateDataChangesLambdas is not suspend anymore. It won't just produce a list of notifications for the UI thread, but an UpdateWork comprising async work and notifications.
• Recycler.update will receive that and decide to do the sync path (no async work) or the coroutine path (async work).
• As both paths need to assign the new recycler data, notify onReady and assign the adapter that is encapsulated in applyNotifications and called from both places.
• The async path still checks that currentUpdate is not changed, and call onCancelled if it changed (some other call to update invalidated us).

[pyricau]

Ship it!

[pyricau]

(one might wonder why no tests were updated in this change, but I'm going to pretend like this thought didn't occur to me)