Remove handlers.HandlerConstConst type

[ScottMansfield]

Repetitive issue title aside, this little change has been bothering me for a while. The NilHandler is a function that returns a HandlerConst (a function itself). It has a parameter that conforms to another type called HandlerConstConst that hasn't really panned out to be useful. It's best to remove that type. While I'm at it I can change NilHandler to be more natural looking by just passing it as a parameter instead of calling it with an empty string, which just looks mysterious.