big performance improvement!

[samuelcolvin]

Change Summary

• Turns out change_exception although elegant, was quite slow.
• other small improvements to how exceptions are structured

roughly 25% improvement on benchmarks.

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.rst file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #819 into master will decrease coverage by 0.06%.
The diff coverage is 97.46%.

@@            Coverage Diff             @@
##           master     #819      +/-   ##
==========================================
- Coverage     100%   99.93%   -0.07%     
==========================================
  Files          17       17              
  Lines        2989     3023      +34     
  Branches      583      582       -1     
==========================================
+ Hits         2989     3021      +32     
- Misses          0        2       +2

Impacted Files	Coverage Δ
pydantic/main.py	100% <100%> (ø)	⬆️
pydantic/types.py	100% <100%> (ø)	⬆️
pydantic/errors.py	100% <100%> (ø)	⬆️
pydantic/datetime_parse.py	100% <100%> (ø)	⬆️
pydantic/utils.py	100% <100%> (ø)	⬆️
pydantic/networks.py	100% <100%> (ø)	⬆️
pydantic/error_wrappers.py	100% <100%> (ø)	⬆️
pydantic/validators.py	99.42% <95.74%> (-0.58%)	⬇️

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