Table lookups instead of enums?
I'm reading in some data that represents certain values using what is essentially a table lookup.  That is, the data may contain data in the range `0..5`, and each value is effectively an index into a list of values, such as `[118, 144, 220, 300, 430, 1200]`.  That is, if the data contains `1`, that corresponds to a value of `144`.

I would love to be able to represent this in kaitai in a manner similar to enums.  Something like:

```
tables:
  frequency:
    0: 118
    1: 144
    2: 220
    3: 300
    4: 430
    5: 1200
seq:
  - id: frequency
    type: u1
    table: frequency
```

The only alternative I see right now is to create a enum-based solution, like:

```
enums:
  frequency:
    0: freq_118
    1: freq_144
    2: freq_220
    3: freq_300
    4: freq_430
    5: freq_1200
```

...but while that leads to better readability, it doesn't actually help all that much because I would still need to maintain an external mapping table to map the enum to an actual value.
