Do not consider proofs with side effects as opaque #601

Job	Run time
install-windows	1h 6m 26s
install-opam (ubuntu-latest, 4.14.x)	11m 34s
build-extension (macos-latest)	1m 31s
build-extension (ubuntu-latest)	52s
build-extension (windows-latest)	2m 17s
dev-setup-opam (ubuntu-latest, 4.14.x)	11m 46s
nix-dev-build (ubuntu-latest)	9m 54s
nix-dev-build (macos-latest)	16m 7s
release-opam (ubuntu-latest, 4.14.x)	11m 35s
	2h 12m 2s

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