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.