Eagerly expand cfg/cfg_attr in pyclasses

[mejrs]

Implements #5125 (comment)

It's just for enum variants and fields in enums/structs for now. We can expand to impl blocks and functions in the future.
It seems to work reasonably well actually, the main ugly part is handling of pyclass/pyo3 options because we need to parse them to figure out where the pyclass macro is. Thoughts on how to do this better are welcome.

There's also some code dealing with forwarding cfgs that we can remove in the future, in particular the check for empty enums is unreachable now I believe.

[codspeed-hq]

Merging this PR will improve performance by 10.74%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 125 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	test_empty_class_init_py	11.7 µs	10.6 µs	+10.74%

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_{Comparing mejrs:cfg_eval (03d922d) with main (5c6807d)}