fn get_array_index(&self, index: i64) -> Result<usize, RubyError> {
        if index < 0 {
            let i = self.len() as i64 + index;
            if i < 0 {
                return Err(RubyError::range("Index too small for array."));
            };
            Ok(i as usize)
        } else {
            Ok(index as usize)
        }
    }
Calculate array index.
if `index` is a zero or positeve integer, return `index`.
Else, return `len` + `index.`
ruruby/src/value/array.rs