Seal some traits inside impl_ submodule

[Cheukting]

Attempt to close #3908

• First, I try sealing PyAddToModule to see if I am getting in the right direction
• @davidhewitt Please kindly review
• If it is correct I can try working on more of the traits

[davidhewitt]

Thanks, this looks exactly like what we were thinking 👍

Should I merge this as-is or wait for further commits?

[davidhewitt]

I had a quick reflection on whether the generic implementations here are a problem; I don't think they are as we control these types and they're hidden zero-sized types with no runtime use.

Note that something like impl<T> Sealed for Vec<T> {} would be a problem as users could then start adding other PyO3 trait implementations to Vec<MyType>, which they could then control and do stuff with.

[Cheukting]

I will have a look again at this, I think we can seal more traits. Will ask for a review again when it is ready.

[Cheukting]

@davidhewitt I have tried sealing most of the traits in pyclass.rs and it seems more complicated than what I did before (involving some macro as well). Do you want to have a look before I move on to other files in impl_? This seems like a big PR with lots of changes.