Should everything be a functor by default?

[CarloLucibello]

As proposed in https://github.com/FluxML/Functors.jl/issues/49 and implemented in https://github.com/FluxML/Functors.jl/pull/51 we can switch the functor semantics from opt-in to opt-out, and eliminate an obscure piece of magic from users' code. Should we do it for v0.14 (hopefully approaching soon)?