[ refactor ] ScopedSnocList: WIP #1199

Job	Run time
initialise	5s
quick-check	5m 57s
ubuntu-bootstrap-racket	3m 5s
ubuntu-bootstrap-chez	2m 9s
macos-bootstrap-chez	1m 59s
nix-bootstrap-chez	2m 59s
ubuntu-self-host-previous-version	34s
windows-bootstrap-chez	4m 34s
ub-test-elab-util	0s
ub-test-collie	0s
ub-pack-test-lsp	0s
ub-pack-test-pack	0s
ub-test-frex	0s
ub-test-katla-and-html	0s
macos-self-host-chez	0s
ubuntu-build-api	0s
ubuntu-self-host-chez	0s
ubuntu-self-host-racket	0s
windows-self-host-racket	0s
	21m 22s

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