WIP: Variant Type

[AdamGS]

Just stashing results here to make the work more visible:

1. Variant DType and Array
2. ParquetVariant encoding
3. VariantGet expression

[codspeed-hq]

Merging this PR will degrade performance by 20.89%

❌ 5 regressed benchmarks
✅ 995 untouched benchmarks
⏩ 1466 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	bench_many_nulls[0.9]	468.5 µs	533.8 µs	-12.24%
❌	Simulation	patched_take_200k_first_chunk_only	4.8 ms	5.4 ms	-10.69%
❌	Simulation	patched_take_200k_dispersed	4.7 ms	5.6 ms	-16.59%
❌	Simulation	take_200k_first_chunk_only	3.3 ms	4.2 ms	-20.89%
❌	Simulation	take_200k_dispersed	3.6 ms	4.5 ms	-19.61%

---

_{Comparing adamg/variant (83828b2) with develop (1bb7e68)}

Footnotes

1. 1466 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[a10y]

do you think we should make this #[non_exhaustive] ?

[AdamGS]

I think adding a new dtype should be very rare, but it is pretty painful to do right now.