Add BigInt gamma method via factorial

[oscardssmith]

Presumably more efficient than exp(loggamma(Complex{BigFloat}))

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (173bff6) to head (aa4b41f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #12   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          306       307    +1     
=========================================
+ Hits           306       307    +1     

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