Use custom `Serialize` implementations for metric types

[alexdewar]

While we are using the erased-serde crate, which should allow us to do this, we don't actually have custom Serialize implementations for MetricTrait types anyway (edit: we do, but we're not using them); we use the value() method instead. We should fix this. In particular, this would be useful for NPVMetric, to make it clear to the user when we are using the fallback comparison because the denominator is zero.

[alexdewar]

Marking this as on hold, because Adam has suggested a new approach for NPV which would only involve one number: #1201

If we do this, then the current approach of having a value() method will be sufficient and we can remove the custom Serialize implementations and the dependency on erased-serde.