add pure string benchmark

[codecov[bot]]

Codecov Report

Merging #56 (0f5b323) into main (b6a645c) will not change coverage. Report is 1 commits behind head on main. The diff coverage is n/a.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #56 +/- ## ======================================= Coverage 95.12% 95.12% ======================================= Files 8 8 Lines 1067 1067 ======================================= Hits 1015 1015 Misses 52 52 ``` ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/pydantic/jiter/pull/56?src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pydantic). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pydantic) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/pydantic/jiter/pull/56?src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pydantic). Last update [71df64a...0f5b323](https://app.codecov.io/gh/pydantic/jiter/pull/56?src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pydantic). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pydantic).

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #56 will not alter performance

_{Comparing x100 (0f5b323) with main (71df64a)}

Summary

✅ 42 untouched benchmarks

🆕 3 new benchmarks

Benchmarks breakdown

	Benchmark	main	x100	Change
🆕	x100_jiter_iter	N/A	4 µs	N/A
🆕	x100_serde_value	N/A	6.5 µs	N/A
🆕	x100_jiter_value	N/A	6.8 µs	N/A