Function bookbinder_common::number_to_letter

pub const fn number_to_letter(n: u8) -> Result<char, ()>

given a number, return the corresponding letter e.g. 0 -> A, 1 -> B, 2 -> C. Returns an error if the number is greater than 25

let number = 1;
assert_eq!(number_to_letter(number), Ok('B'));