Function spsheet::column_to_index

pub fn column_to_index<'a, S>(value: S) -> usize where
    S: Into<Cow<'a, str>>, 

String index to usize index start with 0

use spsheet::*;
assert_eq!(0, column_to_index("A"));
assert_eq!(1, column_to_index("B"));
assert_eq!(25, column_to_index("Z"));
assert_eq!(26, column_to_index("AA"));