Proto,tx_rollup: Store a linked list of inbox levels
This is another version of !4332 (merged) . Actually, !4332 (merged), can be merged and this MR can be considered as a refactorisation.
I have tried to make the separation of concern better between the various modules added for the TORUs. Hopefully, we will have better invariants that can be guaranteed statically.
Edited by François Thiré