implements incompleteStruct

[ringabout]

  type
    DIR* {.importc: "DIR", header: "<dirent.h>", incompleteStruct.} = object
    Dirent* {.importc: "struct dirent", header: "<dirent.h>".} = object
      d_type* {.importc: "d_type".}: uint8
      d_name* {.importc: "d_name".}: array[256, char]

As in Nim, DIR is marked incompleteStruct.

Since DIR is forward declared, we can not access its size. We need to handle the sizeof for it specifically

In Nim, it causes problems for genTypeInfo. And some (mis)usage still produce code gen errors

type
  DIR* {.importc: "DIR", header: "<dirent.h>", incompleteStruct.} = object

let sss = sizeof(DIR)
echo sss

or use DIR as fields of objects without reference, name using DIR by value

In Nimony, it seems that only sizeof concerns incompleteStruct for now, i.e., sizeof(DIR). Since any type using DIR by
value already produces code gen errors. This PR should make sizeof(DIR) use the size of a pointer, while incompleteStruct might be useful in the future

[Araq]

Well sizeof must detect this pragma and return "unknown" (I don't know if it can do that already, but we need this).

[ringabout]

type
  DIR* {.importc: "DIR", header: "<dirent.h>", incompleteStruct.} = object

const s = sizeof(DIR)

With the compat mode, it gives cannot evaluate expression at compile time