Several important changes from algorithmic point of view
(1) bundle/bubble resolution, now, we keep bigger one and eliminate
overlapped smaller one. It is important to keep the duality-symmetry of
the graph in the process.
(2) Switch to more greedy layout but without finding the global highest
score path yet. There is actually some good and simple theory behind why
the greedy layout works and it will not generate mis-assembly for most
of the cases. I will need to document it in more detail someday.

The stats of this code applying on CHM1 testing dataset:

Contig Stats
Max     80735125
Total   2823535076
n50     8266465
n90     816396
n95     257055

The largest contig is > 80Mb mapped to chr2 in one piece.

For the diploid testing data set, while it did not beat the previous
more agressive greedy layout, it gets decent assembly N50 = 2Mb for
ler-0 + col-0 now.