Should everything be a functor by default?

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)? 
