Replaces the call to NotNull with a direct test. Closes #45.

[scottdorman]

Based on #45 (comment)

BTW, perf in this area hasn't come up for years. I personally wouldn't bother authoring these micro-optimizations.

I opted to just change Requires and not worry about the other classes.

For consistency, should I make the same change to vs-validation?

[codecov-commenter]

Codecov Report

Merging #50 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   90.90%   90.94%   +0.03%     
==========================================
  Files           9        9              
  Lines         275      276       +1     
==========================================
+ Hits          250      251       +1     
  Misses         25       25              

Impacted Files	Coverage Δ
src/Validation/Requires.cs	96.33% <100.00%> (+0.03%)	⬆️

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[scottdorman]

Closes #45.

[AArnott]

consistency, should I make the same change to vs-validation?

Yes please