[swift-evolution] Removing "_ in" from empty closures

[Cole Campbell]

> +1. In general, I think we should allow implicit arguments, without requiring the closure to use all the implicit $n variables like we do today. These should all be valid:
> 
> let _: () -> () = {}
> let _: (Int) -> () = {}
> let _: (Int, Int) -> Int = { 5 }
> let _: (Int, Int) -> Int = { $0 }
> let _: (Int, Int) -> Int = { $1 }

+1. This would be excellent.