Add Void, Null and Empty types

[gcanti]

const Void = t.irreducible('Void', x => x === undefined) 
const Null = t.irreducible('Null', x => x === null) 
const Empty = t.irreducible('Empty', () => false) 

[hexpunk]

I'm not sure about the use case for Empty, but :+1: for Void and Null