New graph io (WIP)

Okay, here's some code to read bintsv4 TupleGraphs. There's also code to write graphs.

It currently has big parallel write problems on the Sampa cluster, but should work okay on Pal. Reading works fine on both.

@bholt, you can try using this to run pagerank using /pic/projects/grappa/twitter/twitter-all.bintsv4.

(I have some more work to do to add tsv loading support and multi-file support. Soon!)