support Rust 1.70 as MSRV

[davidhewitt]

Change Summary

This PR sets Rust 1.70 as the minimum supported version. To go lower, we would need to replace use of OnceLock with external once_cell::sync::OnceCell both here and in jiter.

I think going below 1.65 would be hard because we use both GATs and let-else from that release.

Related issue number

Related to #1176

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @samuelcolvin

[codspeed-hq]

CodSpeed Performance Report

Merging #1206 will degrade performances by 23.75%

_{Comparing dh/rust-msrv (63aad1a) with main (47aff70)}

Summary

❌ 3 regressions
✅ 145 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	main	dh/rust-msrv	Change
❌	test_core_future	47.7 µs	55.3 µs	-13.76%
❌	test_core_future_str	27 µs	35.5 µs	-23.75%
❌	test_small_class_core_dict	31.9 µs	38.8 µs	-17.77%

[davidhewitt]

please review

[sydney-runkle]

Looks good overall, just left one question

[sydney-runkle]

Does setting this set the min version, or the required version?

[davidhewitt]

A minimum version, and at the moment all this will do is cause Cargo to fail the build if the installed rust version is lower.

Future Rust versions might do more here (e.g. fail the build if we accidentally use stuff newer than this version).