feat: Memoize readBytes/readText to avoid excess fetches

[effigies]

This treats them the same as loadTSV and loadJSON.

This PR also updates the type signature for filememoize a la #337.

Closes #348.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.02%. Comparing base (2377750) to head (fa42596).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/schema/context.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #349      +/-   ##
==========================================
+ Coverage   87.00%   87.02%   +0.01%     
==========================================
  Files          51       51              
  Lines        3887     3892       +5     
  Branches      624      624              
==========================================
+ Hits         3382     3387       +5     
  Misses        495      495              
  Partials       10       10              

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