Consider computation expressions for callbacks

[kMutagene]

This can either exist in parallel to or instead of the currently used constructors.

so for

  Callback(
      Inputs = [
          ...
      ],
      Output = ...
      State = [...],
      HandlerFunction = ...
      )
  )

we could have

callback {
	inputs [...]
	state [...]
	output ...
	handlerFunction ...
}

with the benefit of keyword-style named parameters instead of constructor arguments