Structural opaque result types

SE-0328 widens the range of places that opaque result types can be used.

For example, we can now return more than one opaque type at a time:

import SwiftUI

func showUserDetails() -> (some Equatable, some Equatable) {
    (Text("Username"), Text("@twostraws"))
}

We can also return opaque types:

func createUser() -> [some View] {
    let usernames = ["@frankefoster", "@mikaela__caron", "@museumshuffle"]
    return usernames.map(Text.init)
}

Or even send back a function that itself returns an opaque type when called:

func createDiceRoll() -> () -> some View {
    return {
        let diceRoll = Int.random(in: 1...6)
        return Text(String(diceRoll))
    }
}

So, this is another great example of Swift harmonizing the language to make things consistently possible.