Support copy, deepcopy, eq on types

[GodTamIt]

This change implements the __richcmp__, __copy__() and __deepcopy__() functions for certain types that can be copied and compared.

[cjrh]

Looks good. Just need python tests on the types with the new impls. Document, Facet and SearchResult right?

[GodTamIt]

I just realized that marking classes as frozen won't work with pickling 🙁 (__setstate__ needs the class to be mutable)

[adamreichold]

I just realized that marking classes as frozen won't work with pickling slightly_frowning_face (__setstate__ needs the class to be mutable)

Since immutability is highly desirable for both performance and correctness, especially in a language like Python built around pervasive shared mutability, I would recommend not dropping it easily to implement the Pickle protocol. (PyO3 is even planning to make frozen classes the default eventually due to this.)

Maybe we can use #96 to take a step back and reconsider why Pickle support was requested? Is full integration with the Pickle protocol necessary or only (de-)serialization from PyBytes which could also be implemented using methods returning newly created immutable instances?

[GodTamIt]

Since immutability is highly desirable for both performance and correctness, especially in a language like Python built around pervasive shared mutability, I would recommend not dropping it easily to implement the Pickle protocol. (PyO3 is even planning to make frozen classes the default eventually due to this.)

I was able to accomplish pickling with __reduce__ and/or __getnewargs__ and pythonize in the change. No mutability required for those.

This change is okay regardless.

[cjrh]

I'm good to move forward, but I'd like to get feedback from @adamreichold too before I merge.