Personally, just stick with the double linked lists, be it via pointers
or array index. Any of these other suggestions either complicate the
algorithms or slow down traversal or both to save that word of memory
and slow down the initial implementation.
These details are easily changed after the initial TUPLE implementation
by replacing the meat of the next/prev iterator. There will be lots of
time for everyone to try their favorite double linked list alternative
before we write TUPLES out to a file in some production compiler and
commit ourselves to specific memory footprint.
As long as the entire thing has a clean interface, changing details like
this is trivial. Whats important is whether the proposal meets what we
expect our future needs to be, such as LTO and such. Have we missed
anything critical...
Andrew