Darwin notification center wrappers

[Absolucy]

Wrappers for CFNotificationCenterAddObserver and friends would be useful, allowing callbacks to easily be made from Rust.

Relevant documentation:

• CFNotificationCenter
• CFNotificationCenterPostNotification
• CFNotificationCenterAddObserver
• CFNotificationCenterRemoveObserver

[Absolucy]

Also, a cfstr! macro would likely be needed, which acts like the CFSTR define.

[abhizer]

Any updates regarding this?