Work around declared but undefined Structs/Unions

[ParadoxV5]

These are:

• opaque structs/unions
  • e.g., struct NAME with no { … }
• Structs with a flexible array member ( flexible array member ffi/ffi#436)
  • Requires Arrays in Struct/Unions should be expanded rather than replaced with pointers like in Functions #14

Without knowing the members, they can only be used #by_ref until manually specifying their actual layouts.