libstore: generatorize protocol serializers

this is cursed. deeply and profoundly cursed. under NO CIRCUMSTANCES
must protocol serializer helpers be applied to temporaries! doing so
will inevitably cause dangling references and cause the entire thing
to crash. we need to do this even so to get rid of boost coroutines,
and likewise to encapsulate the serializers we suffer today at least
a little bit to allow a gradual migration to an actual IPC protocol.

(this isn't a problem that's unique to generators. c++ coroutines in
general cannot safely take references to arbitrary temporaries since
c++ does not have a lifetime system that can make this safe. -sigh-)

Change-Id: I2921ba451e04d86798752d140885d3c5cc08e146

src/libstore/worker-protocol.hh

@@ -122,7 +122,7 @@ struct WorkerProto
 #if 0
     {
         static T read(const Store & store, ReadConn conn);
-        static void write(const Store & store, WriteConn conn, const T & t);
+        static WireFormatGenerator write(const Store & store, WriteConn conn, const T & t);
     };
 #endif
 
@@ -131,9 +131,10 @@ struct WorkerProto
      * infer the type instead of having to write it down explicitly.
      */
     template<typename T>
-    static void write(const Store & store, WriteConn conn, const T & t)
+    [[nodiscard]]
+    static WireFormatGenerator write(const Store & store, WriteConn conn, const T & t)
     {
-        WorkerProto::Serialise<T>::write(store, conn, t);
+        return WorkerProto::Serialise<T>::write(store, conn, t);
     }
 };
 
@@ -219,7 +220,7 @@ inline std::ostream & operator << (std::ostream & s, WorkerProto::Op op)
     struct WorkerProto::Serialise< T > \
     { \
         static T read(const Store & store, WorkerProto::ReadConn conn); \
-        static void write(const Store & store, WorkerProto::WriteConn conn, const T & t); \
+        [[nodiscard]] static WireFormatGenerator write(const Store & store, WorkerProto::WriteConn conn, const T & t); \
     };
 
 template<>