Simplify Fold Implementation

[viceroypenguin]

This PR simplifies the implementation of Fold. The existing FoldImpl is unnecessary and onerous to maintain. Since it receives 18 arguments, it must check which one should be non-null and then it must select that one for Folding. This a) creates additional branches and code bloat, and b) adds an additional method that must be maintained.

It is better to have each overload of Fold do what it needs to do directly. This avoids branching and the Assume.NotNull() check, and simplifies the code significantly.

[codecov]

Codecov Report

Merging #961 (8a7def4) into master (aefa90b) will increase coverage by 0.10%.
The diff coverage is 100.00%.

❗ Current head 8a7def4 differs from pull request most recent head 2eb83f7. Consider uploading reports for the commit 2eb83f7 to get more accurate results

@@            Coverage Diff             @@
##           master     #961      +/-   ##
==========================================
+ Coverage   92.49%   92.59%   +0.10%     
==========================================
  Files         113      113              
  Lines        3424     3432       +8     
  Branches     1024     1049      +25     
==========================================
+ Hits         3167     3178      +11     
+ Misses        192      191       -1     
+ Partials       65       63       -2     

Impacted Files	Coverage Δ
MoreLinq/Batch.cs	98.07% <100.00%> (+3.73%)	⬆️
MoreLinq/Fold.cs	100.00% <100.00%> (+2.12%)	⬆️
MoreLinq/Fold.g.cs	100.00% <100.00%> (ø)

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[atifaziz]

Minor comments other looks good to merge, thanks!

[atifaziz]

Thanks!