Integrate playground into repository

[zwaldowski]

• .playground that ships along Deferred.xcworkspace.
• Fastlane script that takes that .playground and makes it into an independent .playground
• Fastlane action that takes that .playground and makes it into an independent .playgroundbook

[zwaldowski]

See #161

[zwaldowski]

A next step to do this will be to canonicalize the lock-free shims in the absence of the Xcode project/Atomics pseudomodule. Apple's libc ships with the methods we'd need (1, 2, 3), which I've had success with integrating already.

EDIT: These are not (in theory) limited to Apple's libc. A C11-compliant platform should be implementing them as well.

[zwaldowski]

Closing due to inactivity.