Use `Rmath` instead of private `StatsFuns.RFunctions` submodule

[devmotion]

Currently, Distributions uses non-public functions in the private submodule StatsFuns.RFunctions. This could break in any new release of StatsFuns.

This PR switches the code to public functions in Rmath (which was already used by the private functions in StatsFuns.RFunctions).

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 86.35%. Comparing base (bbdd4f1) to head (693630a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2011   +/-   ##
=======================================
  Coverage   86.35%   86.35%           
=======================================
  Files         146      146           
  Lines        8782     8782           
=======================================
  Hits         7584     7584           
  Misses       1198     1198           

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