switch-on string?
Is it possible to switch-on non-integer values? I'd like to do something like this and the compiler complains that mapping values are not allowed:

```
packet:
  seq:
    - id: packet_id
      type: str
      size: 8
      - id: body
        type: 
          switch-on: packet_id
          cases:
            "STRING_A": body_a
            "STRING_B": body_b
            "STRING_C": body_c
  types:
    body_a: # ...
    body_b: # ...
    body_c: # ...
```