Make StaticString conform to Hashable

[Karl]

@Nevin

I am stating the fact that this restriction on protocol conformances was not (to my knowledge) raised during the discussions leading up to the declaration of ABI stability on Apple platforms.

Yes it was. You may have missed it, but Apple were well aware of it (and ABI stability on their platforms is their call to make).

@jrose

// CoreKit overlay in iOS 16
public struct SomeStruct { … }

// A non-retroactive conformance! But one with availability.
// We don't actually support this syntax yet, but we're going
// to need something like it.
extension SomeStruct:
    @available(iOS 16, *) CustomStringConvertible {
  var description: String {
    return "SomeStruct, via CoreKit itself"
  }
}