Add semi-additive categories infrastructure #3911

Job	Run time
opam-build (dev, ubuntu-latest)	2m 15s
quick-build (dev, ubuntu-latest)	2m 17s
quick-build (latest, ubuntu-latest)	2m 16s
opam-build (latest, ubuntu-latest)	2m 16s
opam-build (supported, ubuntu-latest)	2m 16s
quick-build (supported, ubuntu-latest)	2m 16s
nix	2m 16s
build (supported)	2m 17s
build (dev, --warnings)	2m 16s
build (latest)	2m 17s
doc-coqdoc	1s
doc-dep-graphs	1s
coqchk	1s
install	1s
delete-artifacts	6s
doc-alectryon	1s
doc-timing	1s
deploy-doc	1s
	22m 55s

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