Replace working set more efficiently

[djmitche]

When renumbering tasks, we delete and re-add the entire working set:

taskchampion/taskchampion/src/taskdb/working_set.rs

Lines 38 to 42 in e725a38

	if renumber {
	txn.clear_working_set()?;
	for elt in new_ws.drain(1..new_ws.len()).flatten() {
	txn.add_to_working_set(elt)?;
	}

This is pretty slow, and in most cases only a few tasks will change numbers. Optimize for that case, by only updating the items that have changed. This should allow removing the Storage::clear_working_set method as well.

[koleesch]

I would like to do it, but i don't understand the datastructures. So I need help for it.

[djmitche]

Maybe we can come back to this after #410?