Add nthcombo functions

[depial]

This adds two methods nthcombo(a, k, n) and nthcombo(a, combo) which produce the nth lexicographic k-combination and the number n of the provided combination in lexicographic order, respectively. They are intended to complement the implementations already present in permutations.jl.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.30%. Comparing base (b808ce2) to head (f6c8eb2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
+ Coverage   97.17%   97.30%   +0.13%     
==========================================
  Files           8        8              
  Lines         813      854      +41     
==========================================
+ Hits          790      831      +41     
  Misses         23       23              

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