Stricter non-nullable types, renaming and general upkeep

[stargazing-dino]

Hi !

I forked this project for a client app I was working on and decided to make some changes. This is a rather large PR but adds a lot of improvements.

• Complies to the Google naming strategy
• Types that are non-optional will now be marked non-nullable.
• Replaces custom deserialization with json_serializable
• Removes several unused or redundant classes

Note, there are breaking changes in naming but nothing too major