Support declaring functions as integer

This adds a new syntax for declaring functions integer:

    #external integer(<function name>(<arity)).

If a function is declared integer, it may enable some variables to be
detected as integer as well.

include/anthem/AST.h

@@ -149,6 +149,7 @@ struct FunctionDeclaration
 
 	std::string name;
 	size_t arity;
+	Domain domain{Domain::General};
 };
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////

include/anthem/StatementVisitor.h

@@ -190,7 +190,7 @@ struct StatementVisitor
 		const auto fail =
 			[&]()
 			{
-				throw LogicException(statement.location, "only #external declarations of the form “#external <predicate name>(<arity>).” supported");
+				throw LogicException(statement.location, "only #external declarations of the form “#external <predicate name>(<arity>).” or “#external integer(<function name>(<arity>)).” supported");
 			};
 
 		if (!external.body.empty())
@@ -204,6 +204,47 @@ struct StatementVisitor
 		if (predicate.arguments.size() != 1)
 			fail();
 
+		const auto handleIntegerDeclaration =
+			[&]()
+			{
+				// Integer function declarations are treated separately if applicable
+				if (strcmp(predicate.name, "integer") != 0)
+					return false;
+
+				if (predicate.arguments.size() != 1)
+					return false;
+
+				const auto &functionArgument = predicate.arguments.front();
+
+				if (!functionArgument.data.is<Clingo::AST::Function>())
+					return false;
+
+				const auto &function = functionArgument.data.get<Clingo::AST::Function>();
+
+				if (function.arguments.size() != 1)
+					return false;
+
+				const auto &arityArgument = function.arguments.front();
+
+				if (!arityArgument.data.is<Clingo::Symbol>())
+					return false;
+
+				const auto &aritySymbol = arityArgument.data.get<Clingo::Symbol>();
+
+				if (aritySymbol.type() != Clingo::SymbolType::Number)
+					return false;
+
+				const size_t arity = aritySymbol.number();
+
+				auto functionDeclaration = context.findOrCreateFunctionDeclaration(function.name, arity);
+				functionDeclaration->domain = ast::Domain::Integer;
+
+				return true;
+			};
+
+		if (handleIntegerDeclaration())
+			return;
+
 		const auto &arityArgument = predicate.arguments.front();
 
 		if (!arityArgument.data.is<Clingo::Symbol>())

src/anthem/IntegerVariableDetection.cpp

@@ -49,12 +49,9 @@ struct TermDomainVisitor
 		return ast::Domain::General;
 	}
 
-	static ast::Domain visit(ast::Function &)
+	static ast::Domain visit(ast::Function &function)
 	{
-		// Functions may return values of any type
-
-		// TODO: implement explicit integer specifications
-		return ast::Domain::General;
+		return function.declaration->domain;
 	}
 
 	static ast::Domain visit(ast::Integer &)