ORU: Use precise types in place of Z.t

The Z.t type is used everywhere in the ORU implementation. This is a shame, because the Tezos codebase offers a lot of precise types with ad-hoc invariant checking (e.g., a Tez.t cannot be negative).

In this MR, we replace several Z.t with more precise types (we may have even fix a bug by doing so, although this is not totally sure 😅)