reduce universes in FinNat #1957

Job	Run time
opam-build (supported, ubuntu-latest)	4m 53s
quick-build (supported, ubuntu-latest)	3m 19s
nix	2m 25s
opam-build (latest, ubuntu-latest)	4m 57s
build (supported)	3m 31s
quick-build (latest, ubuntu-latest)	3m 17s
opam-build (dev, ubuntu-latest)	4m 32s
build (latest)	3m 22s
quick-build (dev, ubuntu-latest)	3m 21s
build (dev, --warnings)	3m 24s
doc-alectryon	15m 9s
coqchk (supported)	2m 1s
coqchk (latest)	1m 58s
doc-coqdoc	2m 20s
coqchk (dev)	2m 21s
doc-dep-graphs	7m 55s
doc-timing	1m 32s
install (supported)	1m 47s
install (latest)	1m 46s
install (dev)	1m 39s
deploy-doc	0s
delete-artifacts	6s
	1h 15m 35s

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