Make IsTrunc an inductive type #1740

Job	Run time
nix	2m 22s
build (supported)	2m 51s
opam-build (supported, ubuntu-latest)	4m 13s
build (latest)	2m 49s
opam-build (8.17, ubuntu-latest)	4m 18s
quick-build (supported, ubuntu-latest)	2m 37s
build (dev, --warnings)	3m 1s
opam-build (latest, ubuntu-latest)	4m 4s
quick-build (latest, ubuntu-latest)	2m 30s
opam-build (dev, ubuntu-latest)	4m 25s
quick-build (dev, ubuntu-latest)	2m 43s
doc-alectryon	12m 59s
doc-dep-graphs	7m 27s
doc-coqdoc	1m 11s
coqchk (supported)	1m 32s
doc-timing	1m 7s
coqchk (latest)	1m 22s
coqchk (dev)	1m 28s
install (supported)	58s
install (latest)	1m 0s
install (dev)	1m 5s
deploy-doc	0s
delete-artifacts	3s
	1h 6m 5s

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