Sao Tome and Principe IBAN parsing rules

Examples

    iex>  %IbanEx.Iban{
    ...>    country_code: "ST",
    ...>    check_digits: "23",
    ...>    bank_code: "0001",
    ...>    branch_code: "0001",
    ...>    account_number: "00518453101",
    ...>    national_check: "46"
    ...>  }
    ...>  |> IbanEx.Country.ST.to_string()
    "ST 23 0001 0001 00518453101 46"