use hashbrown to speedup python string caching

[samuelcolvin]

targets the dh/py2 branch to avoid future conflicts.

[codecov[bot]]

Codecov Report

Merging #43 (5944b31) into dh/py2 (d98f5ed) will decrease coverage by 0.08%. The diff coverage is 86.66%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dh/py2 #43 +/- ## ========================================== - Coverage 89.53% 89.46% -0.08% ========================================== Files 9 9 Lines 1061 1063 +2 ========================================== + Hits 950 951 +1 - Misses 111 112 +1 ``` | [Files](https://app.codecov.io/gh/pydantic/jiter/pull/43?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pydantic) | Coverage Δ | | |---|---|---| | [src/python.rs](https://app.codecov.io/gh/pydantic/jiter/pull/43?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pydantic#diff-c3JjL3B5dGhvbi5ycw==) | `94.91% <86.66%> (-0.78%)` | :arrow_down: | ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/pydantic/jiter/pull/43?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/43?src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pydantic). Last update [d98f5ed...5944b31](https://app.codecov.io/gh/pydantic/jiter/pull/43?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 #43 will not alter performance

_{Comparing py2-hashbrown2 (5944b31) with dh/py2 (d98f5ed)}

Summary

✅ 37 untouched benchmarks

🆕 2 new benchmarks

Benchmarks breakdown

	Benchmark	dh/py2	py2-hashbrown2	Change
🆕	python_parse_string_array_unique_not_cached	N/A	22.8 ms	N/A
🆕	python_parse_string_array_unique	N/A	10.6 ms	N/A

[davidhewitt]

Superseded by #51