Annotate HexaryTrie.root_node, and remove setter

[carver]

What was wrong?

Fix #103

How was it fixed?

Annotate the return value, update tests that used the raw root node, and add a new test that HexaryTrie().root_node == HexaryTrie().traverse(())

Also removed the setter. Do we really need it? I guess for symmetry, you would want to set with an annotated node.

Finally: Updated the traverse_from() API to take the new annotated node instead of the raw node. It seemed silly to return an annotated node and expect a raw one back as an argument.

TODO

• squash

Cute Animal Picture

[carver]

I think I'll rage-merge tomorrow. It's pretty small, just a couple cleanups, really.