IBAN Validation (mod 97)

An IBAN is validated by moving the first four characters to the end, replacing each letter with two digits (A = 10 ... Z = 35), and checking the resulting number mod 97 equals 1.

Implement valid_iban(s):

• ignore spaces; letters may be any case
• the country code (first 2 chars) must be letters and the check digits (next 2) must be digits
• the length must match LENGTHS[country] for known countries (unknown countries are invalid)
• compute the mod 97 piece by piece without building one giant integer (process a few digits at a time)

Also implement format_iban(s): upper case, grouped in fours separated by spaces.