Increase test coverage

[FreezyLemon]

The code coverage is ~66% at the moment. This could be a lot higher.

The crate implements relatively primitive types, so good tests don't have to be very complex I think. Even "dumb" tests like assert_eq!(plane, plane.clone()) are basically free to implement, so why not.

Some things that are missing tests:

• (de)serialization in general
• Frame<T>
• some of the math fns