core: add metric savings to audit result

[adamraine]

This PR allows audits to indicate the savings the audit has on each metric in the unit of the respective metric. Originally I imagined we would compute the potential 0-1 score increase for each metric, but I think it's better this way for a few reasons:

• It wouldn't be that hard to recompute the log normal score in the report renderer, we would just need to add computeLogNormalScore and the default metric scoring options to the report renderer.
• We will need the metric savings if we decide to compute impact against field metrics in the report renderer.
• We don't have a system for audits to depend on one another, so computing the potential 0-1 score increase would require us to wire all the metric computation data artifacts into every performance audit.