Function fold_digits 

pub fn fold_digits<'a, N, S, I, C>(
    streamed: S,
    acc: N,
    radix: u8,
    neg: bool,
) -> impl Parser<I, Output = (N, usize, bool)> + 'a
where
    N: CheckedMul + CheckedAdd + CheckedNeg + TryFrom<u8> + Clone + 'a,
    S: IterableParser<I, Item = C> + 'a,
    I: Positioned<Ok = C> + ?Sized + 'a,
    C: Character + 'a,

Takes a streamed parser of digits, folds it to an acc as following digits.

The output value consists of a folded result of acc, a number of folded digits and the last item will be true is the number has overflowed.