Permit non-lazy instances
User guide section [4.12 Instances](https://doc.kaitai.io/user_guide.html#_instances_data_beyond_the_sequence) states:

> Another very important difference between the seq attribute and the instances attribute is that instances are lazy __by default__. What does that mean? Unless someone would call that body getter method programmatically, no actual parsing of body would be done.

"By default" could be restated "always" because there is currently no syntax to make instances non-lazy. @generalmimon proposed a workaround to add a `size 0` to the seq like:

```
  - size: 0
    if: "my_first_instance == [].as<bytes>"
```
or
```
  - size: 0
    if: 'my_first_instance != my_first_instance
```

forcing the instance to be evaluated. But it could be more elegant if the KSY syntax supported something like:
```
instances:
  my_first_instance:
    lazy: false
    pos: 0
    size: 8
    type: my_custom_type
 ```

that would force parsing this instance. The rationale is not performance -- in case `my_custom_type` is utilized like a callback, and has a side-effect to invoke a API when this field is parsed, then the caller would want this instance parsed non-lazy.