k76: ported "unitig" and "clean" from fermi1

Although my focus is no assembly, we still need these functionality.
forgot this file
r78: "assemble" working on a tiny example
r79: graph simplification works
r80: fixed a bug in rld0

There are two versions of rld_dec0(). The first is generic. The second works
with DNA only and is unable to decode beyond block boundaries. Therefore, the
second is NOT a replacement of the first version. I forgot this point.