Make random optional

[mborland]

Closes: #198

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.3%. Comparing base (e42dee2) to head (baca67d).

Additional details and impacted files

@@           Coverage Diff           @@
##           develop    #217   +/-   ##
=======================================
  Coverage     97.3%   97.3%           
=======================================
  Files          192     192           
  Lines        14477   14477           
  Branches      5425    5425           
=======================================
  Hits         14078   14078           
  Misses         326     326           
  Partials        73      73           

Files with missing lines	Coverage Δ
include/boost/safe_numbers/random.hpp	100.0% <ø> (ø)

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