Only use RecursiveContainerValidator when necessary

[samuelcolvin]

With this we can set ref on every model etc. and not worry about performance - pydantic-core will only wrap validators in RecursiveContainerValidator when they're referenced elsewhere and therefore need to be stored in slots.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.64%. Comparing base (146309f) to head (8b6cbcc).
Report is 989 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #243   +/-   ##
=======================================
  Coverage   98.63%   98.64%           
=======================================
  Files          49       49           
  Lines        5121     5149   +28     
  Branches       36       36           
=======================================
+ Hits         5051     5079   +28     
  Misses         70       70           

Files with missing lines	Coverage Δ
src/validators/mod.rs	98.48% <100.00%> (+0.14%)	⬆️

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