Disallow Void as type parameter (opt in/out)

I found myself adding the `NotVoid` constraints to every type parameters which is not very convenient. I would prefer disallowing Void as type parameter altogether, with an opt in/out mechanism for people to choose.

p.s. From https://github.com/HaxeFoundation/haxe-evolution/pull/76 I think the direction is to have an unit type and I think we are going to have `haxe.NoData` so I guess it makes sense to disallow Void