Fix 'Option' return of storage

[wuminzhe]

Renew old PR #210

For example

For a storage that has a return type Option<Vec<u8>>, the storage data through RPC will returns:

• 0x00 for Some([])
• 0x080001 for Some([0, 1])

So, if subxt want to use the Option<Vec<u8>> to decode the returns, the hex should be changed to:

• 0x00 to 0x0100
• 0x080001 to 0x01080001

which is prefixed with a 01 for scale codec Some

Or, find a way to use Vec<u8> to decode.

[cla-bot-2021]

User @wuminzhe, please sign the CLA here.

[hackfisher]

Related? #120

[wuminzhe]

After reading the macro code, I feel this PR is not correct.