It should be possible to reset a ZnCharacterReadStream

Run
```Smalltalk 
a := ZnCharacterReadStream on: 'abc' asByteArray readStream.
a next.
a reset.
a next
```
and you will notice it is not possible to reset an instance of ZnCharacterReadStream as the method #reset is not implemented.

Implementation of #reset could be provided as ZnEncodedStream>>reset

```Smalltalk
reset
	stream reset
```

@svenvc