Non-breaking platform-independent random_mod_core

[mrdomino]

This just makes 32-bit behave like 64-bit currently behaves. Performance, and number generation, is identical to master.

[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.89%. Comparing base (e9f0efd) to head (3fc01d4).

Files with missing lines	Patch %	Lines
src/uint/rand.rs	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1023      +/-   ##
==========================================
+ Coverage   79.86%   79.89%   +0.02%     
==========================================
  Files         163      163              
  Lines       17737    17756      +19     
==========================================
+ Hits        14166    14186      +20     
+ Misses       3571     3570       -1     

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