Typed throw functions

[Dante-Broggi]

@Joe_Groff

• rethrows could become first-class, reducing down to just polymorphic throws :

func foo( : () throws -> ()) rethrows // Swift 3
func foo<T: Error>( : () throws T -> ()) throws T // Swift X
func foo<T: Error>(_: () throws T -> ()) throws Either<MyErrors, T>

Note that this equivalence between rethrows and generic thrown types is invalid, due to eg. this function, which is valid today:

struct Err: Error {}
func myMap<T, U> (x: T, f: (T) throws -> U) rethrows -> U {
  do {
    return try f(x)
  } catch {
    throw Err()
  }
}