native: implement ctExists function

[goshawk-3]

As suggested in #208 (comment), we could add a function to read only the byte array length slot. If it's 0, we can assume no byte array exists at that location, reducing unnecessary slot reads for the ciphertext when it's not needed.

func ctExists(api ChainStorageApi, contractAddress common.Address, handle common.Hash) bool {
...
}

• Eventually replace resultCt := ReadBytesToAddress with the new ctExists. This should be considered in the context of the feature for handling SSTORE opcode
• Rename ReadBytesToAddress accordingly