initial cut of RouterService

[cthulhua]

Motivation

Solution

[LucioFranco]

This looks pretty good! I think we should just expose it and I think that should work!

[alce]

Hi, this looks very useful.

Unless I'm mistaken, could we add something like this to the Router's impl instead of the service free function and it should be good to go?

    pub fn into_service(self) -> RouterService<A, B> {
        RouterService { router: self }
    }

[a1ph]

Should it be closed as #419 is merged.