Major redesign of the entire library:

• Root type is now KMLFile rather than KMLDocument.
• Types that used to have properties of any KMLxxx now have type-erased wrapper types like AnyKMLPlacemark or AnyKMLGeometry.
• Initializers have all been updated.
• Random helper functions have been removed -- library now focuses just on initializing and reading KML data.
• Added extensive unit testing.

No further plans for updates anytime soon, but I appreciate any PRs or suggestions. Enjoy!