Add support for foreign constructor overload

Make vm_fun.hpp fun again by moving setters/getters
into a separate header.
Add new make_overloaded_foreign_class(), but code is still
untested and it's a work in progress at this point.
Interface will change.

src/class_manager.cpp

@@ -92,7 +92,7 @@ namespace wren {
 
 		auto it_found = m_classes.find(TempClassName{module_name, class_name});
 		if (m_classes.cend() != it_found)
-			return (*it_found->second)();
+			return it_found->second();
 		else
 			return {nullptr, nullptr};
 	}