Codechange: Use iteration when dealing with all HouseSpecs.

src/newgrf_house.cpp

@@ -52,6 +52,15 @@ std::vector<HouseSpec> &HouseSpec::Specs()
 	return _house_specs;
 }
 
+/**
+ * Gets the index of this spec.
+ * @return The index.
+ */
+HouseID HouseSpec::Index() const
+{
+	return static_cast<HouseID>(this - _house_specs.data());
+}
+
 /**
  * Get the spec for a house ID.
  * @param house_id The ID of the house.