Making `World.topology` accessible without the getter
Improving the interface to `Updater`
Passing lambdas (instead of `Updater`s) to objects that create updates

The goal of this is to be able to (soon) privatize the `World`'s
`Updater`.
Beefing up `ColorModel`
Unifying turtle-creation code

Now, turtles can only be instantiated by the `World`, just like with
patches and links.  As it should be.