Improve opaque error message for Handler::layer

[davidpdrsn]

Layers applied with Hander::layer must implement Clone. If they don't you get the familiar "Handler not implemented ..." error. Other methods like Router::layer doesn't require this and its pretty confusing why a layer would work on a router but not on a handler. I just spent an hour helping someone on Discord track this down 😅

The reason the Handler::layer requires L: Clone is that layers can only be applied to Services and turning a Handler into a Service requires the state. We don't have the state yet in Handler::layer so instead we store the layer with the handler and apply the layer in <Layered as Handler>::call.