make everything hashable again

[amniskin]

Dags, FnDags, Heads and Indexes are the only database items that aren't content addressed. We should make Dags and FnDags content addressed. This means the dags map (in the commit tree) might have many names pointing to the same dag. This also means that you get a new dag any time you mutate a dag (by adding or naming nodes). We should expect plenty of orphaned dags because of this.

[amniskin]

We're also going to want to move the name and message parameters to Repo.commit instead of begin for two reasons (and one side benefit):

1. The dags map update would be transactional.
2. We would otherwise need to keep track of the dag name while we're executing so that we can update the dags map as we add nodes. This complicates the code.
3. Dags could then import from previous versions of themselves (they can't now because the previous version is no longer in the dags map).

The python lib can stay the way it is now (with name and message in the constructor) but submit the name and message with the Dag.commit call.