Expand stirlings1 and stirlings2 to accept Integer

[mschauer]

The documentation for stirlings1 and stirlings2 indicates that they can use BigInts if provided with such as argument. However the existing functions specify n and k arguments to be Int. This patch allows support for BigInt integers without breaking type consistency of return types, and also provides for some memoization, since otherwise the functions, since they are recursive, can be slow when handling very large integers. (@wherrera10 in #84)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.25%. Comparing base (e005ee0) to head (1edd903).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
+ Coverage   97.22%   97.25%   +0.02%     
==========================================
  Files           8        8              
  Lines         829      837       +8     
==========================================
+ Hits          806      814       +8     
  Misses         23       23              

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