Proposal draft for `is case` (pattern-match boolean expressions)

Evolution Pitches
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From alternatives section:

I like a variation of the above alternative, IMHO it deserves consideration:

// given this enum:
enum E {
    case foo(bar: Int, String)
    case qux
}

// let's assume this was autogenerated:
// (see implementation details below)
extension E {
    var foo: (bar: Int, String)! { ... }
    var qux: E! { ... }
}

// then we can write:
var e = E.foo(bar: 1, "hello")
print(e)                    // foo(bar: 1, "hello")
print(e.qux)                // nil
print(e.foo)                // Optional((bar: 1, "hello"))
print(e.foo.bar, e.foo.1)   // 1 hello
print(e.foo?.bar, e.foo?.1) // Optional(1) Optional("hello")
print(e.foo != nil)         // true
print(e.qux != nil)         // false
print(e.foo?.bar != nil)    // true
let (bar, baz) = e.foo
print(bar, baz)             // 1 hello
print(e.qux)                // nil
e.foo.bar = 2   // mutating associated value! 😃 (traps if not foo)
print(e.foo)    // Optional((bar: 2, "hello"))
e.foo?.1 = "world" // mutating associated value by index this time
print(e.foo)    // Optional((bar: 2, "world"))
Implementation details 
// let's assume this is autogenerated:
extension E {
    var foo: (bar: Int, String)! {
        get {
            switch self {
            case let .foo(bar: bar, baz): return (bar: bar, baz)
            default: return nil
            }
        }
        set {
            let val = newValue!
            self = .foo(bar: val.bar, val.1)
        }
    }
    var qux: E! {
        get {
            switch self {
            case .qux: return E.qux
            default: return nil
            }
        }
        set {
            precondition(newValue != nil)
            self = .qux
        }
    }
}

Edited: updated the sample and implementation details

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