Move Tristate class to separate header

The Tristate class (representing truth values that are either true,
false, or unknown) is used at multiple ends. This moves it to a separate
header for reusing it properly.

src/anthem/Simplification.cpp

@@ -363,7 +363,7 @@ struct SimplificationRuleSubsumptionInBiconditionals
 			std::find_if(and_.arguments.cbegin(), and_.arguments.cend(),
 			[&](const auto &argument)
 			{
-				return (ast::equal(predicateSide, argument) == ast::Tristate::True);
+				return (ast::equal(predicateSide, argument) == Tristate::True);
 			});
 
 		if (matchingPredicate == and_.arguments.cend())