Make ExpressionPromoter public + Fix issue with null constant expression compare

[david-garcia-garcia]

A couple of changes in this PR:

• Checks for "null" constant expressions were done against Constants.NullLiteral. This is "wrong" mostly because nulls that did not source in that same literal will fail to compare as this is an instance level comparison. The following will evaluate to false:

Constants.NullLiteral == Expression.Constant(null)

Expression.Constant(null) == Expression.Constant(null)

To avoid confusion, I completely removed the three "Literals" in constants.

I also made ExpressionPromoter public, so that it can be extended from outside the project.

[codecov]

Codecov Report

Merging #266 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #266   +/-   ##
======================================
  Coverage    96.5%   96.5%           
======================================
  Files          41      41           
  Lines        7213    7213           
======================================
  Hits         6961    6961           
  Misses        252     252

Impacted Files	Coverage Δ
...tem.Linq.Dynamic.Core/Parser/ExpressionPromoter.cs	100% <100%> (ø)	⬆️
.../System.Linq.Dynamic.Core/Parser/KeywordsHelper.cs	100% <100%> (ø)	⬆️
...ystem.Linq.Dynamic.Core/Parser/ExpressionParser.cs	98.65% <100%> (ø)	⬆️
...ystem.Linq.Dynamic.Core/Parser/ExpressionHelper.cs	100% <100%> (ø)	⬆️
src/System.Linq.Dynamic.Core/Parser/Constants.cs	100% <100%> (ø)	⬆️

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