Ensure that one-way implicit conversions also work for value types

[alexweav]

My previous PR (#285) fixed a couple issues with one-way implicit conversions, but we still aren't properly checking which type is promotable when comparing value types.

Added some extra logic to fix this. When comparing a value type against a class, we now check for implicit conversions both ways (value type to class, and class to value type). This was already fixed for non-value types, which are handled in a different section of the logic.

Updated the test to hit both the value-type case and the non-value-type case.

[codecov]

Codecov Report

Merging #287 into master will increase coverage by 0.29%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #287      +/-   ##
=========================================
+ Coverage    87.2%   87.5%   +0.29%     
=========================================
  Files          42      42              
  Lines        4266    4280      +14     
=========================================
+ Hits         3720    3745      +25     
+ Misses        546     535      -11

Impacted Files	Coverage Δ
...ystem.Linq.Dynamic.Core/Parser/ExpressionParser.cs	86.58% <100%> (+0.45%)	⬆️
...ystem.Linq.Dynamic.Core/Parser/ExpressionHelper.cs	95.58% <0%> (+1.47%)	⬆️
...rc/System.Linq.Dynamic.Core/DynamicClassFactory.cs	98.63% <0%> (+1.81%)	⬆️

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