Fix certain cases where implicit conversions aren't correctly detected when parsing comparison operators

[alexweav]

Hello, all.

I discovered a few edge cases where implicit conversions defined between two types would not be used when parsing comparison operators:

• The parser only searches for implicit conversions on the type which is being converted, and not the target type. If the conversion is only defined on the target type, it is missed, resulting in a parse failure.
• The parser doesn't check for implicit conversions at all when parsing equality comparisons on non-value types. If both types are non-value types (e.g. class and string), implicit conversions won't be considered at all.

I added a test which reproduces both cases, as well as fixes for these issues. The test now passes with the fixes for both issues applied.

Please let me know how I can improve the fixes here. Thanks!

[codecov]

Codecov Report

Merging #285 into master will increase coverage by 0.15%.
The diff coverage is 88.88%.

@@            Coverage Diff            @@
##           master    #285      +/-   ##
=========================================
+ Coverage   87.04%   87.2%   +0.15%     
=========================================
  Files          42      42              
  Lines        4260    4266       +6     
=========================================
+ Hits         3708    3720      +12     
+ Misses        552     546       -6

Impacted Files	Coverage Δ
...ystem.Linq.Dynamic.Core/Parser/ExpressionParser.cs	86.13% <88.88%> (+0.52%)	⬆️

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[StefH]

Thanks for this PR, can you take a look at my comment?

[alexweav]

Hello @StefH, any updates on this PR?