Base58 does not preserve one-to-one byte correspondence with the
original data, so different combinations of the same number of bytes
might have different encoded string length. We use GAS transfer to mint
HASHY token, where the token hash is Base58Encode(Ripemd160(data + txHash)).
The problem is that `invokescript` RPC call is used to define transfer tx
sysfee, thus, txHash during testinvoke differs from the actual one, that's
why resulting token ID may have different length during testinvoke and
real invoke. As far as we use token ID as a key to store contract
values, the storage price may also differ. The result is failing
TestNEP11_OwnerOf_BalanceOf_Transfer test due to `gas limit exceeded`
error:
```
logger.go:130: 2021-06-10T21:09:08.984+0300 WARN contract invocation failed {"tx": "45a0220b19725eaa0a4d01fa7a6cdaac8498592e8f3b43bdde27aae7d9ecf635", "block": 5, "error": "error encountered at instruction 36 (SYSCALL): error during call from native: error encountered at instruction 22 (SYSCALL): failed to invoke syscall 1736177434: gas limit exceeded"}
executor_test.go:219:
Error Trace: executor_test.go:219
nep11_test.go:132
nep11_test.go:235
Error: Not equal:
expected: 0x2
actual : 0x4
Test: TestNEP11_OwnerOf_BalanceOf_Transfer
```
Fixed by using base64 instead of base58 (base64 preserves the resulting
encoded string length for the same input length).
