() has %Undefined% type

[Necr0x0Der]

I'm not sure whether subj is a bug or a feature, but for me it was somewhat unexpected that the code

(: cns (-> $A $A $A))
!(cns 1 ())

type-checks. This behavior might be convenient, so I don't have strong opinion whether we need to add (: () Unit) into stdlib, but we may need to decide what behavior should be and add a unit test covering it.

[vsbogd]

Adding (: () Unit) doesn't help in current code state. Because any expression can be additionally interpreted as %Undefined% type.