Refactor root package tests

I want to refactor the testing to be more in line with one of the recently added packages, eg, `plainkey`:

1. For `New*` functions, test the returned private values against statically compiled values.
2. For serialization tests, test the serialization against a statically contained value, such as a JSON string or sequence of bytes
3. For deserialization tests, test the deserialization against statically contained values to detect for backwards compatibility issues
4. Add unit tests for other features as usual.