Add picklers and unpickler for Nothing

[jvican]

Sometimes, it is mandatory to have picklers and unpicklers for Nothing. A compelling use case would be:

val s = spore { (x: Int) => i.toString }
s.pickle // picks the default type Spore[Int, String] {type Captured = Nothing}

Currently, the above snippet fails because the picklers are missing. I can see how other use cases would benefit from this addition while not being detrimental at all. The expected behaviour of the picklers is to do, logically, nothing.

[jvican]

Addressed by #433.