Optional explicit `self` parameter declaration in methods

Evolution Pitches
62

This can be demonstrated with the use of default implementations.

protocol P {
    init(value: Int)
    var value: Int { get }
    mutating func increment()
}

extension P {
    mutating func increment() {
        self = Self(value: value + 1)
    }
}

final class C: P {
    let value: Int
    init(value: Int) {
        self.value = value
    }
}

do {
    var x = C(value: 0)
    x.increment()
    x.increment()
    print("C(value: \(x.value))") // prints: C(value: 2)
}

C.increment is a mutating function, even though C is a class.