Function flat_takec

pub fn flat_takec<I, O, E, C, F>(
    input: I,
    len: C,
    parser: F,
) -> IResult<I, O, E>
where
    I: Input,
    E: ParseError<I>,
    C: ToUsize + Copy,
    F: Parser<I, Output = O, Error = E>,

Take len bytes from input, and apply parser.