Function encode_scalar_tail 

fn encode_scalar_tail(
    encode_table: &[u8; 64],
    input: &[u8],
    output: &mut [u8],
    input_index: usize,
    output_index: usize,
) -> usize

Scalar encode of input[input_index..] into output[output_index..], resuming from a 3-byte group boundary. Returns the total number of output bytes written.