automatic field dereference allows arbitrary depth

automatic dereference is deep, as I was surprised to find out; is that documented? is that even a good feature?
```nim
type A = ref object
  a0: int
type B = ptr A
proc main() =
  var a = A(a0: 1)
  var b = a.addr
  var c = b.addr
  echo c[][][].a0
  echo c.a0 # works but should it?
main()
```