Revert "Make Game an abstract singleton"

This reverts commit f5554865759bc4227ee5f1a56d5f5932aae15698.
Added ability to load and unload chunks.
Remove unecessary Optional from provideUnchecked in ServiceManager
Fix a multitude of Javadoc formatting errors.

For how I did this see https://gist.github.com/gratimax/47dffe6b764ac5757e3b.