bigint: Replace dynarray usage with a slice::Buf.

[briansmith]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 97.18310% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.48%. Comparing base (75dc1c3) to head (bfb78d2).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/polyfill/uninit_slice.rs	93.54%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2850      +/-   ##
==========================================
+ Coverage   96.46%   96.48%   +0.02%     
==========================================
  Files         204      203       -1     
  Lines       20382    20337      -45     
  Branches      520      518       -2     
==========================================
- Hits        19662    19623      -39     
+ Misses        601      595       -6     
  Partials      119      119              

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