Fixes #9562 by adding MGLCompassButton.visibility (and, well, MGLCompassButton).

Changes

• MGLMapView.compassView is now an MGLCompassButton, a subclass of UIImageView (its former type).
• Refactored most of the compass logic out if MGLMapView and into MGLCompassButton.
• Added MGLOrnamentVisibility constants, with options for adaptive, visible, and hidden. These can later be adopted by other ornaments, should we so choose.

To-do

• Finish writing documentation.
• Write new tests, adapt old ones.
• Rename MGLCompassButton to MGLCompassView?
• Consider whether or not to allow arbitrary compasses that aren’t necessarily map subviews.
  • Pushing this to potential tailwork.
• Changelog.