[interfaces Coper & DeepCopier] Enforce sub structures to implement them

[andersfylling]

It's currently troublesome to specify Copier and DeepCopier for every substructure. Write a dependency tree to identify which structs needs the different interfaces instead of requiring var _ Copier = (*Message)(nil)

[andersfylling]

Maybe all exported types should automatically implement this.

[switchupcb]

Consider using Copygen for copier functionality without the use of reflection.