FFI::Struct handles nested structs and nested arrays much better, and avoids
duplicating logic about structure alignment (which it probably does more
correctly that CStruct)
However it's awkward to use in other ways. e.g. no accessor methods;
no proper #inspect; no ntohl for in_addr; no zero-sized arrays at end
of struct; no hooks to convert int32 <-> IPAddr as far as I can see.
