Use nat_iter for iteration #1605

Job	Run time
nix	3m 41s
opam-build (supported, ubuntu-latest)	7m 7s
build (supported)	4m 37s
opam-build (8.17, ubuntu-latest)	5m 48s
quick-build (supported, ubuntu-latest)	3m 24s
build (latest)	3m 39s
opam-build (latest, ubuntu-latest)	5m 9s
quick-build (latest, ubuntu-latest)	3m 55s
build (dev, --warnings)	3m 43s
opam-build (dev, ubuntu-latest)	5m 47s
quick-build (dev, ubuntu-latest)	4m 38s
doc-timing	1m 9s
doc-coqdoc	1m 10s
doc-dep-graphs	8m 48s
doc-alectryon	19m 50s
install (supported)	1m 27s
coqchk (supported)	1m 51s
install (latest)	1m 10s
coqchk (latest)	1m 36s
install (dev)	1m 16s
coqchk (dev)	1m 47s
deploy-doc	0s
delete-artifacts	9s
	1h 31m 41s

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