Remove state generic from graphs

[samuelcolvin]

This is passing except docs which I will update once we've agreed it's a good idea.

this is a step towards storing graph state in a database. My realization was that previously the graph had two things which defined state:

• ctx.state
• the node that was just run or is about to be run

If we get rid of ctx.state:

• we remove one generic from basically everywhere graphs or nodes are defined
• state is now encapsulated in just one object - the node
• we have less weirdness around how to serialize/snapshot state

The point is, if you want a "state" object has we had before, you just need to pass it between nodes, which was happening under the hood anyway.

I think this makes graphs less complicated and less magic without adding significantly more ceremony or boilerplate.

@dmontagu let's discuss when you're online.

[Finndersen]

But doesn't this mean that now nodes have a state attribute that may contain content that's not even relevant/required by that node, and there's no real clear distinction between that state and the nodes other state attributes?