Sets the marshaller of this.
The `marshal_data` of marshal provides a way for a meta marshaller to provide additional information to the marshaller.
For GObject's C predefined marshallers (the `g_cclosure_marshal_*()` functions), what it provides is a callback function to use instead of this->callback.
See also: set_meta_marshal
| this |
a Closure |
| marshal |
a ClosureMarshal function |