Replace definitions Vec with OnceLock slots

[davidhewitt]

Change Summary

This PR reworks the way definitions work so that instead of storing them in a global Vec they are shared inside Arcs with a OnceLock internally meaning that there is one opportunity to build them.

The hope is that this transformation may pave the way for reusing SchemaValidator inside core schema as a compiled subgraph to reduce memory consumption and simplify the defs transformations needed in pydantic.

Related issue number

Related to pydantic/pydantic#7617

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

[codecov]

Codecov Report

Merging #992 (5f14973) into main (1a966d5) will increase coverage by 0.05%.
The diff coverage is 85.16%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #992      +/-   ##
==========================================
+ Coverage   92.98%   93.04%   +0.05%     
==========================================
  Files         106      106              
  Lines       15827    15701     -126     
  Branches       35       35              
==========================================
- Hits        14717    14609     -108     
+ Misses       1103     1085      -18     
  Partials        7        7              

Files	Coverage Δ
src/py_gc.rs	100.00% <100.00%> (ø)
src/serializers/extra.rs	99.08% <ø> (-0.03%)	⬇️
src/serializers/mod.rs	98.47% <100.00%> (-0.02%)	⬇️
src/serializers/shared.rs	79.88% <100.00%> (ø)
src/serializers/type_serializers/dataclass.rs	92.72% <100.00%> (ø)
src/serializers/type_serializers/definitions.rs	93.93% <100.00%> (+0.18%)	⬆️
src/serializers/type_serializers/model.rs	94.55% <100.00%> (ø)
src/validators/any.rs	100.00% <100.00%> (ø)
src/validators/arguments.rs	99.60% <100.00%> (-0.01%)	⬇️
src/validators/bool.rs	100.00% <100.00%> (ø)
... and 43 more

... and 1 file with indirect coverage changes

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[codspeed-hq]

CodSpeed Performance Report

Merging #992 will improve performances by 14.1%

_{Comparing dh/faster-definitions (5f14973) with main (1a966d5)}

Summary

⚡ 1 improvements
✅ 137 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	dh/faster-definitions	Change
⚡	test_generator_rust	35.5 µs	31.1 µs	+14.1%

[davidhewitt]

I've pushed a follow-up commit which removes validator cloning, to close #843

[davidhewitt]

please review

[adriangb]

Given the lack of changes to tests this looks good to me!