Change [filter_weapon] to not depend on attack_type::weapon_specials

This changes it to use the function that attack_type::get_value already uses,
thus removing a call to attack_type::weapon_specials. The latter is being
refactored by newfrenchy83, and questions about the effect of that refactor on
this code delayed the review on his PR.

This code is part of the object used for [filter_weapon]formula=, however the
implementation of that filter creates exactly one attack_type_callable, so
doesn't trigger this code. For testing I called it via a debugger.

src/formula/callable_objects.cpp

@@ -162,7 +162,20 @@ int attack_type_callable::do_compare(const formula_callable* callable) const
 		return att_->range().compare(att_callable->att_->range());
 	}
 
-	return att_->weapon_specials().compare(att_callable->att_->weapon_specials());
+	const auto self_specials = att_->specials().all_children_range();
+	const auto other_specials = att_callable->att_->specials().all_children_range();
+	if(self_specials.size() != other_specials.size()) {
+		return self_specials.size() < other_specials.size() ? -1 : 1;
+	}
+	for(std::size_t i = 0; i < self_specials.size(); ++i) {
+		const auto& s = self_specials[i].cfg["id"];
+		const auto& o = other_specials[i].cfg["id"];
+		if(s != o) {
+			return s.str().compare(o.str());
+		}
+	}
+
+	return 0;
 }
 
 unit_callable::unit_callable(const unit& u) : loc_(u.get_location()), u_(u)