be: let a bank account carry its country and read or write its iban - #46

Slovak banks write accounts as IBANs, and a Czech and a Slovak account can share digits and a bank code, which BankAccount could not tell apart. Groundwork for ČSOB SK and Tatra banka.

• country(), Country CZ or SK, an enum of its own, first in the record; part of equality, so the same digits in two countries are two accounts.
• parse(text, country) reads a local form. There is no default country: a local form does not say it, and a default would quietly read a Slovak account as Czech. The one-argument parse is gone; every call, in the parsers, tests and the Spring Boot caller test, now names CZ (58 calls).
• parseIban(text), with or without spaces, checks ISO 13616's modulo 97 and then the same modulo 11 rule as the local form; only CZ and SK. iban() writes it back; a local form and its IBAN compare equal.
• Both countries share the numbering and the modulo 11 weights: checked on a real Slovak IBAN from a public ČSOB SK sample, which passes both checks (the sample's anonymised payer IBAN passes neither).

Breaking, fine before 1.0: BankAccount gains a component and loses parse(String).

Tests: the ČNB sample account both ways, the Slovak IBAN, round trip, two countries apart, bad check digits, a modulo 11 failure behind valid check digits, a German IBAN. They caught one real bug on the way: the IBAN's parts were first passed in IBAN order (bank code first) to a constructor that takes the prefix first. Breaking the modulo 97 or the modulo 11 check each fails a test. make build: green.