SIMD refactor

[samuelcolvin]

Prepare to add x86 SIMD implementation.

This just moves code and does not change any logic whatsoever.

[codecov]

Codecov Report

❌ Patch coverage is 98.46154% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/jiter/src/simd/fallback_string.rs	97.36%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[codspeed-hq]

Merging this PR will degrade performance by 9.76%

⚠️ 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
❌ 3 regressed benchmarks
✅ 66 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	massive_ints_array_jiter_iter	564.1 µs	685.1 µs	-17.66%
❌	python_parse_massive_ints_array	1.1 ms	1.4 ms	-16.66%
❌	massive_ints_array_jiter_value	743 µs	847.4 µs	-12.32%
⚡	sentence_jiter_iter	7.6 µs	6.9 µs	+10.19%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing simd-refactor (a370548) with main (785cf75)}