Create a new Finalization Witness.
A finalization witness is an object whose sole role is to
broadcast when it is garbage-collected. Once the witness is
created, call method its method |forget()| to prevent the
broadcast.
| aTopic | The topic that the witness will broadcast using Services.obs. |
| aString | The string that the witness will broadcast. |
| An object with a single method |forget()|. |