Function wnaf_form 

pub(crate) fn wnaf_form<S: AsRef<[u8]>>(
    wnaf: &mut [Digit],
    c: S,
    bit_len: usize,
    window: usize,
) -> usize

Fills wnaf with the wNAF representation of a little-endian scalar, and returns the number of digits written.