Fix BigInteger

[shargon]

Fix #432 fix #431

[chenzhitong]

Fixed #432 👍

[cloud8little]

@shargon you can add the UT in #434

[shargon]

@cloud8little I think that it works but not for hex string, for hex string we should use Atoi, it's hard to convert System.String,System.Globalization.NumberStyles into our base argument

else if (src.tokenMethod == "System.Numerics.BigInteger System.Numerics.BigInteger::Parse(System.String,System.Globalization.NumberStyles)")
                {
                    ConvertPushNumber(10, null, to);        // Push Base
                    Convert1by1(VM.OpCode.SWAP, src, to);   // Swap arguments
                    Insert1(VM.OpCode.SYSCALL, "", to, BitConverter.GetBytes(ApplicationEngine.System_Binary_Atoi));
                    return 0;
                }

[cloud8little]

@shargon I know that this is implementation problem. but still it is very confusing, because we have

1. (BigInteger)xx
2. BigInteger.Parse()
3. xx.ToBigInteger()

I find that ToBigInteger also work well for hex byte array.

        public static BigInteger testtoBigInteger()
        {
            return ((new byte[] { 0x00, 0x00, 0x8d, 0x49, 0xfd, 0x1a, 0x07 })).ToBigInteger();
        }

neo> invoke 0x3c49e708254d317a85adf34e6d34a265859ee049 testtoBigInteger []
Invoking script with: 'EMAfDBB0ZXN0dG9CaWdJbnRlZ2VyDBRJ4J6FZaI0bU7zrYV6MU0lCOdJPEFifVtS'
VM State: HALT
Gas Consumed: 0.0112428
Result Stack: [{"type":"Integer","value":"2000000000000000"}]
Relay tx(no|yes): no

[shargon]

@cloud8little I agree, please open a new issue in order to find a more user friendly way