xP: implement tab completion

Currently it only goes for the longest common prefix.

Refactor WebSocket handling into an abstraction for our protocol.

The Go code generater finally needed fixing.

xC-proto

@@ -104,6 +104,8 @@ struct EventMessage {
 		} items<>;
 	case BUFFER_CLEAR:
 		string buffer_name;
+
+	// Restriction: command_seq is strictly increasing, across both of these.
 	case ERROR:
 		u32 command_seq;
 		string error;