from_binary.rs

    pub fn from_binary(s: &str) -> Option<u32> {
      s.chars().rev().enumerate().try_fold(0, |acc, (i, c)| {
        match c {
          '1' => Some(acc + 2u32.pow(i as u32)),
          '0' => Some(acc),
          _   => None,
        }
      })
    }