Refactor Subsets core into an iterator method

[atifaziz]

This PR refactors the implementation of Subsets into a simpler iterator method. As a by-product, it:

• fixes Subsets enumerator doesn't return same array from Current #1047
• removes redundant allocations when called via Subsets() overload

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (a263661) 92.52% compared to head (2a72577) 92.49%.

Files	Patch %	Lines
MoreLinq/Subsets.cs	97.14%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1052      +/-   ##
==========================================
- Coverage   92.52%   92.49%   -0.04%     
==========================================
  Files         113      113              
  Lines        3413     3385      -28     
  Branches     1058     1057       -1     
==========================================
- Hits         3158     3131      -27     
+ Misses        189      187       -2     
- Partials       66       67       +1     

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