(Re)introduce `Invariant` trait #6780

Job	Run time
benchcomp-tests	3m 9s
documentation	1m 56s
perf	30m 34s
regression (macos-13)	24m 39s
write-json-symtab-regression	13m 43s
regression (ubuntu-20.04)	21m 48s
regression (ubuntu-22.04)	22m 5s
	1h 57m 54s

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