Passing the current position to a `process` directive
Moved from https://github.com/kaitai-io/kaitai_struct/issues/1195, original by @adrianherrera:

>When I declare a `process` directive (specifically a custom `process`), is there any way I can pass the current position to the `process` function?
>
> If not, do you think this is something that makes sense to implement? I had a quick look, and I think it may be possible to define a new `Identifier` in (`io/kaitai/struct/format/Identifier.scala`), e.g. `val POS = "pos"`. Then in your .ksy file, if you had:
>
> ```yaml
> id: foo
> size: 10
> process: do_something(pos, ...)
> ```
>
> The `pos` argument would be translated to the Kaitai Stream's get position function. E.g. in for Python this would translate to:
>
> ```py
> _process = DoSomething(self._io.pos())
> self.foo = _process.decode(10)
> ```
>
> What do you think? Do you think this makes sense? I want to do this because I want to do some custom processing that depends on the current position of the stream, but I'm not sure if this is too niche and not really a general feature that would actually be useful.