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 #1047
• removes redundant allocations when called via Subsets() overload

[codecov[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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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