Rework parameters/returns

[robrix]

They should be inputs/outputs of a special graph identifier. We would unfortunately have to communicate the initial type to be constrained & solved for extrinsically.

[robrix]

Identifiers are no longer a thing, but it’d still be nice to find a graph’s returns and parameters under a specific “blessed” index.

We could use endIndex, except that we’d have to do an O(n) search through the edges and update any pointing at it every time we add a node.

[robrix]

An alternative would be to replace Node.Parameter and Node.Return with Node.Graph. This would give us a place to encapsulate the parameters, the returns, the type, even the name.