Node/Connection deserialization

Deserialization for nodes and connections! I only did some basic testing
but everything seems to be good.

Attributes are pretty FUBAR and we definitely need to refactor how they
work in the future, but as far as I know they're not necessary for our
MVP.

Constraints are also still pending.
For the purposes of error detection, what happens if valueString is not something like "true" or "false"? Is there a way to check for this case and throw an exception? (I don't think `valueOf` throws if it can't convert the argument.)