Finalize classes

As a breaking change, classes should be finalized to reduce the chance of future breakage by a user extended a class in a way that was never intended.

https://github.com/rectorphp/swiss-knife has a handy tool to handle this, considering parent classes, etc.

Start by reviewing the repository's PHP classes and the linked rectorphp/swiss-knife tool, paying attention to parent classes and inheritance. Done means the intended classes are finalized consistently, with the breaking-change scope understood and inheritance handled.

Written by the indexing model from the issue text.