Add NullSubstitution support to QueryExtentions

[TylerCarlson1]

Solution for Issue #506

Changed ResolvedExpression to go through a list of classes that could possibly resolve it.
Took normal 2 conditions and made own class and added one for null substitution.

If no issues happen throws exception saying can't convert query mapper.

[jbogard]

So...I just did a big refactoring in this area as well, I won't be able to take the PR as-is but should get it most of the way there. I like your refactoring better - it matches how the runtime versions are.

[jbogard]

Nevermind, I get what you're doing. So now we can combine expression result converters - if you have NullSubsitute AND MapFrom, they stack on top of each other. Nice!

[TylerCarlson1]

Yea I'm not aware of any other custom type mappers that might fall under here, but I think it is set up in such a way that we can account for all scenarios to map Expression to Expression and can use it in other places at a later time.

For example #528 for resolving the property maps and null substitutions.
But not right now I think. I can't imagine someone would have a need to do a comparison between a DTO and EF or NHibernate object, by their primary keys, where they would need null substitution.

[jbogard]

Basically there's two levels - value resolvers and mappers. You've got now the equivalent of a value resolver (which includes redirection and null replacement). Those are the big ones right now.

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