LibIPC+IPCCompiler: Templatize encoding/decoding of Optional<T>

This was the last one! IPCCompiler no longer has any type-specific
encoding/decoding logic! :^)

Libraries/LibIPC/Encoder.h

@@ -76,6 +76,15 @@ public:
         return *this;
     }
 
+    template<typename T>
+    Encoder& operator<<(const Optional<T>& optional)
+    {
+        *this << optional.has_value();
+        if (optional.has_value())
+            *this << optional.value();
+        return *this;
+    }
+
     template<typename T>
     void encode(const T& value)
     {