add a constructor to the Category interface #1202

Job	Run time
initialise	5s
quick-check	5m 38s
macos-bootstrap-chez	6m 31s
nix-bootstrap-chez	18m 58s
ubuntu-bootstrap-racket	12m 27s
ubuntu-bootstrap-chez	6m 52s
ubuntu-self-host-previous-version	10m 23s
windows-bootstrap-chez	23m 49s
macos-self-host-chez	20m 7s
ubuntu-self-host-chez	10m 35s
ubuntu-build-api	4m 18s
ub-test-elab-util	46s
ub-pack-test-pack	12m 32s
ub-pack-test-lsp	15m 27s
ub-test-frex	3m 51s
ub-test-collie	23s
ub-test-katla-and-html	5m 35s
ubuntu-self-host-racket	0s
windows-self-host-racket	10m 38s
	2h 48m 55s

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