Adding `ISafe` interface for `CompatibilityFallbackHandler` #425

Job	Run time
verify (verifyOwners.sh)	17m 53s
verify (verifySafe.sh)	11m 12s
verify (verifyModules.sh)	6m 33s
verify (verifyNativeTokenRefund.sh)	2m 37s
verify (verifySignatures.sh)	5m 56s
	44m 11s

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