use ConstantRNGs

[cscherrer]

Since we factored FixedRNG into https://github.com/cscherrer/ConstantRNGs.jl, we should use that instead of duplicating code

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.17%. Comparing base (3d1084f) to head (fc8737c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #150      +/-   ##
==========================================
+ Coverage   53.97%   54.17%   +0.19%     
==========================================
  Files          43       42       -1     
  Lines        1219     1209      -10     
==========================================
- Hits          658      655       -3     
+ Misses        561      554       -7     

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