Tidy up rng functions, towards a real interface

[richfitz]

Merge after #117, contains those commits

This PR starts building a free-function interface to the RNG, covering a few distributions (the ones used in the SIR helper only). The main source of pain is thinking about how to cope with the multiple-draw case vs single draw case, which I have done with a separate function for each

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.59%. Comparing base (2b46639) to head (53839b8).
Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #118      +/-   ##
==========================================
+ Coverage   99.57%   99.59%   +0.02%     
==========================================
  Files          73       73              
  Lines        5582     5711     +129     
==========================================
+ Hits         5558     5688     +130     
+ Misses         24       23       -1     

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