implement custom "Prototype" layer type

[subversive-owl]

i.e., something that should allow a user to pass a closure taking a Vec<_> as input, returning something like a LayerType::Custom that satisfies the LayerConfig requirements.

[drahnr]

Maybe the name Prototype would be better, since the performance will be sub-par, and cause at least two copies/sync operations to/from the compute device.

[subversive-owl]

Prototype is a great name!