Use () -> Void for closures with no parameters that return nothing

[rayfix]

This seems to be the standard these days. () -> (), (Void) -> () are not preferred.

[ISosnovik]

() -> Void is prefered

[JessyCatterwaul]

I think this rule should follow from two other rules:

1. Don't use (Void) as a closure input; just use ().
2. Use Void instead of () for closure outputs.
   Especially with Swift 3's enforcement of parentheses for inputs, I think this adds clarity.

*I do think Void may be better as a tuple element than (). e.g.

let ƒ: (Void, Int) -> Void = {_, int in}

let void: Void = ƒ( (), 1 )
ƒ(void, 2)

I have never felt a desire to use something like this, but that doesn't mean it might not have a use, and it compiles. I like that signature better than this one:

let ƒ: ( (), Int ) -> Void = {_, int in}

[rcritz]

Added.