message: Add support to BigUint.from_str_radix() for using _ as a visual separator
diff: @@ -243,6 +243,7 @@ impl Num for BigUint {
  b'0'...b'9' => b - b'0',
  b'a'...b'z' => b - b'a' + 10,
  b'A'...b'Z' => b - b'A' + 10,
  + b'_' => continue,
  _ => u8::MAX,
  };
  if d < radix as u8 {
  