Reduce the number of extension files

From @mhdostal:

There are a lot of extension files with only a few methods in them. I think it would be more efficient and readable if the extensions were included in the main class file, separated by a //MARK: comment to keep things organized.

Agreed. It can get painful at times. I think there is value as a teaching tool to keep the files super-simple, but I also know I've taken it too far at times. There are some extensions I could definitely roll into the main class file. Let me tackle this in a branch.