Add Empty struct

Evolution Discussion
3

Thanks for providing these links, I've missed this information.
I suppose it is better to wait for User-defined tuple conformances.

@Slava_Pestov can you share some details on progress of tuple conformances? Is it expected in Swift 5.10 / 5.11 / later...?

Some more questions:

  1. Currently this is valid: let instance: Void = (()) // empty tuple with empty tuple
    Will it be possible to do extension Tuple<Void>: Hashable or extension Void: Hashable {}? Are these declarations equivalent?
  2. Will this warning be eliminated?
func buildInstance<T>(_ builder: () -> T) -> T { builder() }
let instance = buildInstance { Void() } // warning: Constant 'instance' inferred to have type '()', which may be unexpected
  1. Will Void, (Void, Void) and (Void, Void, Void) have the same hashValue?