Adds phone_book test to assert if inserted items exist in the phone book.
Adds stub phone_book.get_nr().
Updates phone_book.get_nr() test.
Updates phone_book to use unordered_map to store phone numbers.
Refactors phone_book.
Adds phone_book.has_nr() test.
Implements stub phone_book.has_nr() method.
Adds phone_book.has_nr() test that asserts if method returns true when contact is found.
Implements phone_book.has_nr().
Refactors code.
Updates phone_book.get_nr() to be const.