serializer: Move skip() to serializer.hh

It is part of the core API, like serialize() and deserialize().

serialization_visitors.hh

@@ -86,9 +86,4 @@ size_type read_frame_size(Input& in) {
     return sz - sizeof(size_type);
 }
 
-template<typename T>
-inline void skip(seastar::simple_input_stream& v, boost::type<T>) {
-    return serializer<T>::skip(v);
-}
-
 }

serializer.hh

@@ -91,6 +91,11 @@ inline T deserialize(Input& in, boost::type<T> t) {
     return serializer<T>::read(in);
 };
 
+template<typename T>
+inline void skip(seastar::simple_input_stream& v, boost::type<T>) {
+    return serializer<T>::skip(v);
+}
+
 template<typename T>
 size_type get_sizeof(const T& obj);