Simplify EquiZip implementation

[viceroypenguin]

The various Zip* methods currently share implementation in .ZipImpl(). However, the code is simpler for each one when they are written separately; that is, in this case, the variances are more than the commonality. As such, we should allow each method to be implemented separately.

This PR updates the implementation of EquiZip to be done separately from .ZipImpl().

[viceroypenguin]

NB: This may be a v4.0.0 PR?

[codecov]

Codecov Report

Merging #902 (5ff2ad4) into master (60ec000) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #902      +/-   ##
==========================================
+ Coverage   92.41%   92.43%   +0.02%     
==========================================
  Files         112      112              
  Lines        3426     3425       -1     
  Branches     1017     1019       +2     
==========================================
  Hits         3166     3166              
+ Misses        199      198       -1     
  Partials       61       61              

Impacted Files	Coverage Δ
MoreLinq/EquiZip.cs	100.00% <100.00%> (+4.08%)	⬆️
MoreLinq/ZipImpl.cs	97.77% <0.00%> (-2.23%)	⬇️

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[atifaziz]

Possibly a duplicate of PR #715 by @Orace?

[viceroypenguin]

Duplicate in the sense that both separate the implementations of the three zip functions, yes. However, #715 uses T4 to generate separate implementations of each method for argument count 2-4. Also, #715 and #902 have very different implementations. IMO #902 is simpler, because the control flow is more linear, rather than 4-way branch with multiple exit points.

[Orace]

Some thoughts about this:

• The used pattern probably wastes resources. It's implemented for 4 sources, and for other cases, it uses dummy sources and discards values with a lambda (resulting to memory allocation that can be avoided):

  MoreLINQ/MoreLinq/EquiZip.cs

  Line 71 in 5ff2ad4

  return EquiZipImpl(first, second, Enumerable.Repeat(default(object?), int.MaxValue), Enumerable.Repeat(default(object?), int.MaxValue), (a, b, _, _) => resultSelector(a, b), 2);

  
  It will not scale well if we want to add overloads with more sources.

• The control flow of Zip refactoring #715 can be simplified a little bit to match Simplify EquiZip implementation #902 that offers more complete exception messages.

[viceroypenguin]

@Orace Here's a copy of the code gen for a simpler control flow if you want to steal it.

Closing this PR in favor of an updated #715.