Make use of intern! macro for attribute names used internally

[adamreichold]

Since we do use several attribute names internally, I looked into whether some of them could be candidates for interning. Personally, I do not think this is obvious as while identifiers like __name__ will most likely already be part of Python's string interner dictionary, they do not seem to be used in a hot path of PyO3.

I therefore split this into separate commits to solicit feedback on whether we want to include any of them:

1. __qualname__ used in PyType::name. Here, I am not sure how often production code using PyO3 actually calls this.
2. __all__ and __name__ used in the implementation of PyModule. This seems to be limited to initialization, but the strings are almost surely interned already by other code.
3. The the attribute names used in getattr calls emitted by the FromPyObject derive macro. I think this has the largest possible benefit as this could end up being used in every Python-to-Rust call. But I am also not sure whether enabling this unconditionally is a good idea. (Since the names are given via code and not generated dynamically, this should be ok?)

[davidhewitt]

Sneaky idea that should be nice for performance! It makes me a little uneasy if we know we might have to regress this for PEP 489 combatibility later, however it's still a nice win for the meanwhile.

[birkenfeld]

Related idea: add a comment to getattr/setattr recommending intern!?

[adamreichold]

Related idea: add a comment to getattr/setattr recommending intern!?

Add a comment to the methods of Py and PyAny. Would be glad for feedback on the wording. I think linking the PyDict example given at intern!'s docs is sufficient or should I add a getattr-specific one?

[birkenfeld]

I think an example would not go amiss.

[adamreichold]

I think an example would not go amiss.

Added a single example to each of the four cases.

[birkenfeld]

Thanks a lot!