Rename Deferred to IO to reflect that its an I/O monad.

[zoj613]

Also use destructive substituion where needed to make function signature return types more explicit.

[codecov]

Codecov Report

Attention: Patch coverage is 84.50704% with 11 lines in your changes missing coverage. Please review.

Project coverage is 97.60%. Comparing base (00ade96) to head (684ef4b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
zarr/src/storage/memory.ml	61.53%	5 Missing ⚠️
zarr/src/storage/zip.ml	54.54%	5 Missing ⚠️
zarr-lwt/src/storage.ml	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
- Coverage   97.60%   97.60%   -0.01%     
==========================================
  Files          13       13              
  Lines        1546     1544       -2     
==========================================
- Hits         1509     1507       -2     
  Misses         37       37              

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