Refactor codecs module.

[zoj613]

This commit joins multiple files under the codecs directory into a single codecs.ml file

[codecov]

Codecov Report

Attention: Patch coverage is 99.09502% with 4 lines in your changes missing coverage. Please review.

Project coverage is 97.63%. Comparing base (94822ca) to head (db2446f).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
zarr/src/codecs.ml	99.09%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #86      +/-   ##
==========================================
- Coverage   97.64%   97.63%   -0.01%     
==========================================
  Files          16       13       -3     
  Lines        1569     1563       -6     
==========================================
- Hits         1532     1526       -6     
  Misses         37       37              

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