Conditional valid check
Hi,

Thanks for your awesome work on Kaitai. I was wondering the possibility to conditionally check the validity of a field based on an other field. Let's say :
```yaml
seq:
  - id: kind
    type: u4
    enum: item_kind
  - id: size
    type: u4
    valid : ????
enums:
  item_kind:
    0: kind1
    10: kind2
    20: kind3
```
How can I check the validity of size of according to kind ? What I would like is to express : "if kind == kind1 then size == 12; if kind == kind2 then size = 6; else no check".

Right now I am using TLV so size has a different type for each kind, but I think this is a bit overkill.

Thank you for your help !