Scaffolder: implement the handler for packed chunks

This patch implements a new handler for packed chunks. Instead of
transmitting a DNA sequence as k-mers, it is transmitted as a packed
array of nucleotides with 2 bits per nucleotide.

Reviewed-by: Élénie Godzaridis <telomerika@gmail.com>
Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>