Unexported embedded interfaces break impl

[nyarly]

⮀ impl 'msg diffRezMessage' logging.LogMessage
unrecognized interface: eachFielder

Where logging.LogMessage is like:

	// A LogMessage has structured data to report to a log (c.f. Deliver)
	LogMessage interface {
		DefaultLevel() Level
		Message() string
		eachFielder
	}

	eachFielder interface {
		EachField(FieldReportFn)
	}