LazyBuffer::get_at (+ simplify Combinations::nth)

[kinto-b]

As requested in #914 (comment), I've added a helper for returning the 'current' combination.

I also realised that there's no reason to delegate from .nth to .next() when n==0 so I junked it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.45%. Comparing base (6814180) to head (68df645).
Report is 54 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #917      +/-   ##
==========================================
+ Coverage   94.38%   94.45%   +0.06%     
==========================================
  Files          48       48              
  Lines        6665     6866     +201     
==========================================
+ Hits         6291     6485     +194     
- Misses        374      381       +7     

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[Philippe-Cholet]

Away from the keyboard, I thought that from_ methods usually are constructors (returns Self) which is not the case here. So get_at is a better name than my suggested from_indices.

I'll merge this once the requested change is done, thanks.