cppforliving

Not that I particularly need this, I just spotted https://github.com/tower-rs/tower/issues/636 so I looked at tower-service API and noticed this minor asymmetry. I really like the idea of adding blanket impl...

Any thoughts on this?

There seems to be one subtle difference between `ServiceFn` and blanket impl for async fns. `ServiceFn` has unconstrained `Debug` impl while async fns and closures returning futures do not implement...