- Remove `Info` from `Source`. This method was not used.
- Remove `Stack` from `EvalSource`. This method was not used.
- Remove `Type` and `URN` from `Step`. These values are available via
`Res().URN.Type()` and `Res().URN`, respectively. This removes the
possibility of inconsistencies between the type, URN, and state of the
resource associated with a `Step`.
- Remove URN from StepEventMetadata.