PARTIAL: add list to lwtreslib
Lwtreslib.List centralises Stdlib.List, Lwt_list, and Error_monad (the list traversal part).
Note: this MR includes only the new code. It doesn't make any use of it. That follow up work is in !2245 (merged)
NOTE: This MR shouldn't be merged. Instead !2245 (merged) (a strict superset of this one) should be merged. This one is provided for granularisation only.
Depends on !1895 (merged) !2244 (merged) !2243 (merged)
Part of !1872 (closed)
Edited by Raphaël Proust