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Most of what we interact with in configgraph is other parts of the evaluator that automatically get memoized by patterns like OnceValuer, but the value for a resource instance is always provided by something outside of the evaluator that won't typically be able to use those mechanisms, and so the evaluator's ResourceInstance.Value implementation will now provide memoization on behalf of that external component, to ensure that we end up with only one value for each resource instance regardless of how that external component behaves. In the case of the current planning phase, in particular this means that we'll now only try to plan each resource instance once, whereas before we would ask it to make a separate plan for each call to Value. For now this is just retrofitted in an minimally-invasive way as part of our "walking skeleton" phase where we're just trying to wire the existing parts together end-to-end and then decide at the end whether we want to refactor things more. If this need for general-purpose memoization ends up appearing in other places too then maybe we'll choose to structure this a little differently. Signed-off-by: Martin Atkins <mart@degeneration.co.uk>