Revise & take advantage of pattern matching

[atifaziz]

This PR revises some expressions to take advantage of pattern matching.

[codecov]

Codecov Report

Merging #866 (16368b3) into master (d684bf8) will not change coverage.
The diff coverage is 71.42%.

❗ Current head 16368b3 differs from pull request most recent head 1b1b602. Consider uploading reports for the commit 1b1b602 to get more accurate results

@@           Coverage Diff           @@
##           master     #866   +/-   ##
=======================================
  Coverage   92.45%   92.45%           
=======================================
  Files         108      108           
  Lines        3446     3446           
  Branches     1025     1025           
=======================================
  Hits         3186     3186           
  Misses        197      197           
  Partials       63       63           

Impacted Files	Coverage Δ
MoreLinq/Partition.cs	98.24% <0.00%> (ø)
MoreLinq/FallbackIfEmpty.cs	88.67% <50.00%> (ø)
MoreLinq/Flatten.cs	97.50% <100.00%> (ø)
MoreLinq/Split.cs	89.70% <100.00%> (ø)
MoreLinq/Subsets.cs	94.11% <100.00%> (ø)
MoreLinq/ToDataTable.cs	96.87% <100.00%> (ø)

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