SE-0307: Allow interchangeable use of CGFloat and Double types

It may not have been intended, but that is how it came across out in the wild. And it resulted in a lot of things switching away from Double to use CGFloat under the perception that “something” must be better about it. (Hypotheses included performance, future‐compatibility with 128‐bit, etc.)

If Double is the real recommendation, and this proposal helps consolidate things toward that direction, then I support it.