to_str() -> to_string()

to_str() was changed to to_string() everywhere after https://github.com/rust-lang/rust/pull/15493

In iban.rs to_str() still works (and to_string() breaks the program) but I haven't been able to understand why
you're right, removing it... I added it because I was trying to make sure I was looking at the right type when rustc was complaining about not finding into_str() on vec, and then I forgot to remove it again.

When I find a chain of method calls, sometimes I add typing information just to verify my intuition on the final type that gets assigned to the variable (i.e. if it compiles, the type is the one I thought). This is where a good IDE would be very handy... you hover on a variable and the IDE confirms the type to you....