Add event name, callback support and base64 syscalls

[shargon]

Close #301 #299

• Updated neo nuget
• Added event name
• Added callback syscalls
• Added base64 syscalls

Require neo-project/neo#1723

[ProDog]

I have tested notify, it's OK.

Contract:

        public delegate void mydelegate(params object[] arg);

        [DisplayName("event")]
        public static event mydelegate Notify;

        [DisplayName("TestEvent")]
        public static event Action<byte[], BigInteger> OnEvent;

        public static bool NotifyTest()
        {
            OnEvent(new byte[] { 0x89, 0x77, 0x20, 0xd8}, 1);

            Notify("str", 11, 12);

            Runtime.Log("test");

            return true;
        }

events in abi:

"events":
    [
        {
            "name":"event",
            "parameters":
            [
                {
                    "name":"arg",
                    "type":"Array"
                }
            ]
        },
        {
            "name":"TestEvent",
            "parameters":
            [
                {
                    "name":"arg1",
                    "type":"ByteArray"
                },
                {
                    "name":"arg2",
                    "type":"Integer"
                }
            ]
        }
    ]

Notify:

 "notifications": [
            {
                "contract": "0x90ca09ef53cb3753a3770119b31bf211d410dad5",
                "event_name": "TestEvent",
                "state": {
                    "type": "Array",
                    "value": [
                        {
                            "type": "Buffer",
                            "value": "iXcg2A=="
                        },
                        {
                            "type": "Integer",
                            "value": "1"
                        }
                    ]
                }
            },
            {
                "contract": "0x90ca09ef53cb3753a3770119b31bf211d410dad5",
                "event_name": "event",
                "state": {
                    "type": "Array",
                    "value": [
                        {
                            "type": "Array",
                            "value": [
                                {
                                    "type": "ByteString",
                                    "value": "c3Ry"
                                },
                                {
                                    "type": "Integer",
                                    "value": "11"
                                },
                                {
                                    "type": "Integer",
                                    "value": "12"
                                }
                            ]
                        }
                    ]
                }
            }

[ProDog]

I found contract execute FAULT when the contract uses static variables and delegate event in this PR.
The #309 is same problem.

My contract:

        public delegate void mydelegate(params object[] arg);

        [DisplayName("event")]
        public static event mydelegate Notify;

        private static byte[] myAddr = "NNB8GKS7mdMXXGsAwvXYyEGonkEjDbqNkG".ToScriptHash();

        public static byte[] test()
        {
            return myAddr;
        }

Invoke result:

neo> invoke 0xe7a5322430588a0eb7969100c52a5d8d7bf9642c test
Invoking script with: '10c00c04746573740c142c64f97b8d5d2ac5009196b70e8a58302432a5e741627d5b52'
VM State: FAULT
Gas Consumed: 0.0100889
Result Stack: []
Engine faulted.
Error: insufficient balance.

Please take a look @shargon .

[shargon]

Could you take a look @lightszero ?

[ShawnYun]

We'd better merge it as soon as possible, so we can solve other problems which wait for it.