refactor: remove a bunch of useless "let open Memo.O in" #28903

Job	Run time
Nix (macos-latest)	1m 55s
Format	1m 1s
Nix (ubuntu-latest)	57s
Build microbenchmarks	2m 18s
Build (macos-latest, 5.1.x)	13m 20s
Coq 8.16.1	7m 14s
Coverage (5.1.x)	9m 46s
Create local opam switch (macos-latest, 5)	5m 33s
Build (ubuntu-latest, 5.1.x)	8m 53s
Create local opam switch (macos-latest, 4.14)	5m 35s
Build (ocaml-variants.5.3.0+trunk, ubuntu-latest, true)	5m 0s
Create local opam switch (ubuntu-latest, 5)	4m 55s
Documentation	50s
Build (4.14.x, ubuntu-latest, true)	1m 41s
Build monorepo benchmark docker image	3m 41s
Create local opam switch (ubuntu-latest, 4.14)	4m 55s
Build (4.14.x, macos-latest, true)	2m 40s
Build (5.1.x, macos-13, true)	4m 10s
Build (ocaml-base-compiler.4.14.2,system-msvc, windows-latest)	16m 12s
Build (ocaml-base-compiler.5.1.1,system-mingw, windows-latest)	10m 26s
Build (4.08.x, ubuntu-latest, true)	1m 38s
Build (4.04.x, ubuntu-latest, true, true)	4m 49s
Build (4.02.x, ubuntu-latest, true)	4m 4s
	2h 1m 33s

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