SE-0103: Make non-escaping closures the default

I agree with this proposal. In my own code I’ve found non-escaping parameters to be far and away the most common I create, with only the very occasional block that could escape.

Most times when I’ve seen escaping blocks in the wild it’s in Apple’s frameworks — which makes sense, because Apple’s written GCD and there’s no point in all of us duplicating it.

-Wil