Use new uniform traversors

Context

The recently merged / soon to be merged Lwtreslib work gives access to uniform LWt-, Error- and Lwt-Error-aware traversors for collections. Consequently, we can now replace many ad-hoc traversals (typically Blah.Table.fold (fun acc x -> acc >>=? fun () -> f x) return_unit blah) by more principled traversals (Blah.Table.iter_es f blah).

These transformations, depending on the specific case, (1) simplify the code, (2) avoid an intermediate representation as a list of promise, (3) ensure a sound error/exception management, or (4) a combination of the above.

On top of !1895 (merged) (i.e., this MR is actually only the few commits at the top)
Part of !1872 (closed)

Manually testing the MR

Consider the diff and see local improvements.

Automatic testing

Regular tests should still pass.

Reviewers

Early reviews probably by @mbouaziz and @Saroupille
Before-merge reviews by anyone