Function len_lcp 

pub fn len_lcp<T>(
    a: &[u8],
    m: &Range<T>,
    b: &[u8],
    n: &Range<T>,
) -> GenericResult<T>
where
    T: PrimInt + NumCast,
    Range<T>: Iterator<Item = T>,

Get length of the longest common prefix bits for the given two slices.