Destructors are not inherited

devel branch, --gc:arc

`=destroy` is not invoked when it's inherited.

```nim
import streams

type
  A = ref AObj
  AObj = object of RootObj
    io: Stream
  B = ref object of A
    x: int

proc `=destroy`(x: var AObj) =
  close(x.io)
  echo "closed"
  
var x = B(io: newStringStream("thestream"))
```

No output.
