Callbacks: Manual Clone and Debug impl; Visibility fix

[lpotthast]

Great to see, that this is now part of Leptos :). This PR mainly adds manual Clone and Debug implementations.

The manual Clone impl removes the Clone type bounds on the generic In and Out types. All callback types can be cloned by simply cloning the inner Rc. The generic types are not important here.

The Debug impls are debatable, as they are not strictly needed. But I would say it's good practice to supply users with a Debug impl for ALL custom types a library provides, as this is somewhat expected. (The ability to derive Debug on your types without problems.) If not provided, users would always need to add custom Debug impls for all structs in which they want to store some Callback type and have a Debug impl on that struct.

Also adds a missing pub modifier to construct ViewCallback::new.

[gbj]

Thanks, I always forget about manual Clone impls and it always bites me. And I agree re: Debug implementations, so thanks for those.