Special syntax for blocks that capture all variables as weak

[anandabits (Matthew Johnson)]

@gwendal.roue

We need to convey both the weak references and the guards that bail out whenever a single one of the weak captures has turned nil. We're after a combo.

This topic has been discussed periodically over the years. Here are links to a couple threads on the topic: Guarded closures and [Draft] Guarded closures and `@guarded` arguments.