Add "IsInnerRing" method

Inner ring nodes check their presence in inner ring list during
startup. "IsInnerRing" method allows to check presence efficiently
by calling this method.

neofs_contract.go

@@ -44,6 +44,7 @@ func Main(op string, args []interface{}) interface{} {
 		- Deposit(params: pubKey, amount)                - deposit GAS to the NeoFS account
 		- Withdraw(params: withdrawCheque)               - withdraw GAS from the NeoFS account
 		- InnerRingUpdate(params: irCheque)              - change list of inner ring nodes
+		- IsInnerRing(params: pubKey)                    - returns true if pubKey presented in inner ring list
 		- Version()                                      - get version of the NeoFS smart-contract
 
 		Params:
@@ -53,7 +54,6 @@ func Main(op string, args []interface{}) interface{} {
 		                   contains inner ring signatures
 		- irCheque       - serialized structure, that confirms new inner ring node list;
 		                   contains inner ring signatures
-
 	*/
 
 	ctx := storage.GetContext()
@@ -265,6 +265,26 @@ func Main(op string, args []interface{}) interface{} {
 		putSerialized(ctx, "UsedVerifCheckList", c)
 
 		return true
+	case "IsInnerRing":
+		if len(args) != 1 {
+			panic("isInnerRing: wrong arguments")
+		}
+
+		key := args[0].([]byte)
+		if len(key) != 33 {
+			panic("isInnerRing: incorrect public key")
+		}
+
+		irList := getSerialized(ctx, "InnerRingList").([]node)
+		for i := range irList {
+			node := irList[i]
+
+			if util.Equals(node.pub, key) {
+				return true
+			}
+		}
+
+		return false
 	case "Version":
 		return version
 	}