[ffi] Support passing and returning composites (structs) by value.

I wanted to writing bindings for libclang (so that I can generate bindings from C/C++ header files automatically), but I discovered that libclang API uses structs passed and returned by value.

typedef struct {
  unsigned int_data[4];
  void *ptr_data;
} CXToken;

typedef struct {
  const void *data;
  unsigned private_flags;
} CXString;

CINDEX_LINKAGE CXString clang_getTokenSpelling(CXTranslationUnit, CXToken);

So I can’t really bind to this API. There are some interesting design problems to resolve: e.g. when a structure is returned by value who manages the memory of that struct and how do we prevent people from making a mistake of passing address of that struct to some other method.