SE-0365: Allow implicit self for weak self captures, after self is unwrapped

[cal (Cal Stephens)]

There was a pitch from last August to do that using a syntax like [guard self] or [weak(guard) self]. The Core Team weighed in with this feedback:

@John_McCall

The core team talked about this, and we're reluctant to add features that introduce new sources of control flow unless they significantly improve expressive power.

The idea for this proposal to allow implicit self in weak self closures did spin off from that thread though :)