Add model for vstd Set and prove its axioms #6376

Job	Run time
fmt	57s
smoke-test-and-release-linux	6m 20s
smoke-test-and-release-windows	8m 20s
smoke-test-and-release-macos-x86	10m 56s
test-and-release-macos	7m 0s
test-and-release-macos (record-history)	2m 57s
test-and-release-macos (no-std)	1m 46s
test-and-release-macos (no-alloc)	2m 57s
test-and-release-macos (singular)	2m 50s
test-and-release-macos (cvc5)	2m 21s
release	0s
	46m 24s

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