Transaction decoding: tokens - allow fetching symbol, totalSupply, balances and allowances

[raulk]

Consider the following selection of an ERC20 transaction:

{
  block(number: 1234) {
    transactions {
       decoded {
         ... on ERC20Transfer {
           symbol // new
           totalSupply // new
           from {
             address
             tokenBalance // new
             allowance(spender: "0x1923...") // new
           }
           to {
              address
           }
           value
         }
       }
    }
  }
}