func prefixFieldClashes(data Fields, fieldMap FieldMap) {
	timeKey := fieldMap.resolve(FieldKeyTime)
	if t, ok := data[timeKey]; ok {
		data["fields."+timeKey] = t
	}
	msgKey := fieldMap.resolve(FieldKeyMsg)
	if m, ok := data[msgKey]; ok {
		data["fields."+msgKey] = m
	}
	levelKey := fieldMap.resolve(FieldKeyLevel)
	if l, ok := data[levelKey]; ok {
		data["fields."+levelKey] = l
	}
}
This is to not silently overwrite `time`, `msg` and `level` fields when
dumping it. If this code wasn't there doing:

logrus.WithField("level", 1).Info("hello")

Would just silently drop the user provided level. Instead with this code
it'll logged as:

{"level": "info", "fields.level": 1, "msg": "hello", "time": "..."}

It's not exported because it's still using Data in an opinionated way. It's to
avoid code duplication between the two default formatters.
package logrus
import "time"
const defaultTimestampFormat = time.RFC3339
type Formatter interface {
	Format(*Entry) ([]byte, error)
}