Rename core

[samuelcolvin]

• schema_types.py -> core_types.py
• Split Schema into CoreSchemaStrings and CoreSchema
• Create an new CoreSchemaCombined which replaces Schema

This should make types types easier to work with in pydantic.

[codecov-commenter]

Codecov Report

Merging #248 (ce0dce6) into main (313f287) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #248   +/-   ##
=======================================
  Coverage   98.58%   98.58%           
=======================================
  Files          50       50           
  Lines        5165     5166    +1     
  Branches       36       36           
=======================================
+ Hits         5092     5093    +1     
  Misses         73       73           

Impacted Files	Coverage Δ
pydantic_core/core_schema.py	100.00% <100.00%> (ø)
src/validators/mod.rs	99.63% <100.00%> (ø)

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