Graphbuild could use optimized representation

[jakebiesinger]

@JavierJia @Nan-Zhang
Currently, graphbuild uses and outputs VKmer keys and VKmer edges. They could instead use Kmer keys and a single letter for each neighbor.

[Nan-Zhang]

@jakebiesinger , actually the graphbuilding currently already used Kmer to represent the key of intermediate results(after keyvalueparser, and local aggregater), and the final output's key is still VKmer. So you mean we also could change the key of final results of graphbuliding to Kmer?

[jakebiesinger]

By "a single letter for each neighbor", I meant that given your Kmer and a single letter and direction, you could derive your neighbor's kmer. So during intermediate stages, graphbuild could use this altered representation.