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.
Oh right, I forgot that this was there. Never mind, that's probably the right place for it then.