Adding `ISafe` interface for `CompatibilityFallbackHandler` #402

Job	Run time
verify (verifyOwners.sh)	20m 48s
verify (verifySafe.sh)	10m 3s
verify (verifyModules.sh)	22m 11s
verify (verifyNativeTokenRefund.sh)	4m 6s
verify (verifySignatures.sh)	5m 28s
	1h 2m 36s

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