Even more ?Sized bounds

[wackbyte]

Overview:

• GhostBorrow/GhostBorrowMut for &(GhostCell<T>, ...): The last type is ?Sized.
  • The extra macro I wrote to help with this is duplicated between the two modules since it's really small. I can move it to a new module if needed, though.
• GhostBorrowMut for (&GhostCell<T>, ...): Each type is ?Sized.
  • Didn't require any other changes.
• GhostBorrowMut for [&GhostCell<T>; N]: T is ?Sized.
  • I think I've found a solution to the problems discussed in your previous comment.
  • I made check_distinct generic over a type T and made it take [*const T; N] instead of [*const (); N].
    Each pointer is cast to *const () during comparison (I made sure and checked, and std recommends this for by-address equality of fat pointers; see the last example here).
    [&GhostCell<T>; N] and [*const T; N] definitely have the same layout, so no strange pointer casting issues here.
  • It doesn't require the pointer metadata API!
• I made a few basic positive and negative tests.

[matthieu-m]

This is looking really good. I just think it would be worth being a little more explicit on the assumptions making check_distinct safe in the presence of fat pointers; I suggested one, but if you can think of others please do add them.

As far as I know, the assumptions hold at the moment, so we're in the clear for now, but I find it better to document them should the lay of the land change in the future.