Feature: Added support for implicit type conversions

[arjenvrh]

Added support for implicit type conversions (i.e. a class defines an implicit casting operator to the type of a ConstantExpression).

public class ImplicitCastingClass
{
    public Guid Id { get; set; }
    public static implicit operator Guid(ImplicitCastingClass id)
    {
        return id.Id;
    }
}

[codecov]

Codecov Report

Merging #166 into master will increase coverage by 0.26%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
+ Coverage   82.38%   82.64%   +0.26%     
==========================================
  Files          37       37              
  Lines        3627     3642      +15     
  Branches      488      491       +3     
==========================================
+ Hits         2988     3010      +22     
  Misses        501      501              
+ Partials      138      131       -7

Impacted Files	Coverage Δ
...ystem.Linq.Dynamic.Core/Parser/ExpressionParser.cs	77.79% <100%> (+0.45%)	⬆️
src/System.Linq.Dynamic.Core/Parser/TypeHelper.cs	88.2% <0%> (+0.47%)	⬆️
...c/System.Linq.Dynamic.Core/Tokenizer/TextParser.cs	98.03% <0%> (+1.3%)	⬆️

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[StefH]

Can you also add some unit-test to cover this new code?