[TODO] activation fn registry

[HyperCodec]

While the most widely-used activation functions are predefined, it'd be better to have some sort of registry so that the serde deserialization works with custom activations.

It would also be good to note that there should be a separate flag or something for those activation functions which do not want to be randomly mutated to/fro.

[HyperCodec]

hmm this seems impractical to implement. delaying.

[HyperCodec]

Might implement this using a trait or something and then just boxing it. The current Arc<dyn Fn> model could be very easily replaced by Arc<dyn Activation> and then blanket impl on Fn for some default trait stuff.