Extend support for Equal in trusted func framework

[sonalmahajan15]

Equal support was limited to only handling slice length comparison (e.g., Equal(0, len(s))). This PR extends this Equal support also handle nil comparisons (e.g., Equal(nil, err)). Specifically, this PR makes the following changes:

• Refactors existing requireComparators code to accommodate the extension
• Add support for nil comparison in requireComparators
• Adds test cases for nil comparison
• Adds NotEqual in trusted funcs list and specific test cases for the slice comparison case as well

[Closes #64 ]

[codecov]

Codecov Report

Merging #75 (9a5bbf7) into main (9ab92c9) will increase coverage by 0.09%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
+ Coverage   89.23%   89.32%   +0.09%     
==========================================
  Files          53       53              
  Lines        8016     8048      +32     
==========================================
+ Hits         7153     7189      +36     
+ Misses        716      712       -4     
  Partials      147      147              

Files	Coverage Δ
assertion/function/assertiontree/trusted_func.go	74.33% <86.66%> (+7.87%)	⬆️

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[yuxincs]

This LGTM module a few nits that do not necessarily have to be fixed 😃 .