repeat-expr can't use fields defined in instances
Trying to compile this .ksy file:
```ksy
meta:
  id: floats
  endian: le
instances:
  - id: num_floats
    type: u4
  - id: floats
    type: f8
    repeat: expr
    repeat-expr: num_floats
```
yields the following error:
```
/instances: expected map, got List(Map(id -> num_floats, type -> u4), Map(id -> floats, type -> f8, repeat -> expr, repeat-expr -> num_floats)) (class scala.collection.immutable.$colon$colon)
```

This file is a copy-paste from the examples replacing a `seq` with `instances`.

I'm not sure if it is related to issue kaitai-io/kaitai_struct#512 or not.