Union Serialization

[samuelcolvin]

Continuation from #327, incorporating the ideas from the union discussion there.

@tiangolo @dmontagu @adriangb, I'd love your feedback.

Apart from some coverage and fixing the weird seg fault that's currently occurring in tests, I think this is ready to go.

Todo:

• literal serialiser that warns on invalid value
• warnings on TypedDict serialiser when required fields are omitted, and extra field are included - perhaps behind a check flag.

[codspeed-hq]

CodSpeed Performance Report

Merging #357 union-serialization (58582d6) will not alter performances.

Summary

🔥 0 improvements
❌ 0 regressions
✅ 80 untouched benchmarks

🆕 0 new benchmarks
⁉️ 0 dropped benchmarks

[codecov-commenter]

Codecov Report

Merging #357 (3f8a3cf) into main (8f42a9a) will decrease coverage by 1.44%.
The diff coverage is 79.86%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #357      +/-   ##
==========================================
- Coverage   96.50%   95.07%   -1.44%     
==========================================
  Files          87       90       +3     
  Lines        9455    10038     +583     
  Branches        4        4              
==========================================
+ Hits         9125     9544     +419     
- Misses        330      494     +164     

Impacted Files	Coverage Δ
pydantic_core/__init__.py	100.00% <ø> (ø)
src/errors/mod.rs	92.30% <ø> (ø)
src/errors/value_exception.rs	94.56% <ø> (+2.67%)	⬆️
src/serializers/config.rs	92.94% <ø> (ø)
src/serializers/type_serializers/nullable.rs	74.46% <0.00%> (-22.76%)	⬇️
src/serializers/type_serializers/recursive.rs	84.78% <0.00%> (-12.72%)	⬇️
src/serializers/type_serializers/with_default.rs	77.77% <0.00%> (-19.45%)	⬇️
src/validators/custom_error.rs	100.00% <ø> (ø)
src/serializers/type_serializers/dict.rs	87.77% <41.17%> (-5.73%)	⬇️
src/serializers/type_serializers/url.rs	85.10% <45.45%> (-2.99%)	⬇️
... and 41 more

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