Allow implicit `self` for `weak self` captures, after `self` is unwrapped

[DevAndArtist (Adrian Meister)]

While this is a cool idea, I kinda wish we could tackle a more general idea of rebinding self.

@Joe_Groff

We could steal an idea from Kotlin and let you bind self in the scope of a closure:

let dateFormatter = with(DateFormatter()) { self in
    dateStyle = .short
    timeStyle = .long
}

which seems like a reasonable extension of the much-demanded guard let self = self rebinding idea.