…a Node's result to a stored `previous_result`.

… Nodes ran, and having Nodes include a Context.id in their output.

… after we dirty a Node.

TODO: Storing these in a Vec on the state is potentially less efficient
than storing them on the edges in the graph, since that avoids a bunch of
extra allocation.

… to newly computed generations.

…e outputs.

…gerly prune edges for cleared entries, and lazily prune edges for dirtied entries.

…duler already accounts for this, and it seems reasonable to not bake retry into the Graph.