Support iteration in value instances
```yaml
meta:
  id: test
  file-extension: test
seq:
  - id: bytes
    type: u1
    repeat: expr
    repeat-expr: 10
instances:
  sums:
    value: '_index == 0 ? bytes[0] : sums[_index-1] + bytes[index]'
    repeat: expr
    repeat-expr: 4
```
```invalid key found in value instance, allowed: doc, doc-ref, enum, if, value```