Simplify `prelude-annotate-todo`, make it evaporate when modified.

Ideally, it would be better if the modifying overlay evaporates. I
didn't see a direct way to make that work. With the help of Shingo
Fukuyama's wonderful ov.el package, manipulating overlays becomes much
easier than ever . The "evaporating" idea was stolen from the package's
example code.