Add unconvert linter

This linter checks for unnecessary type convertions.

Some convertions are whitelisted because their type is different
on 32bit platforms

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

events/events.go

@@ -26,9 +26,9 @@ func (e *Envelope) Field(fieldpath []string) (string, bool) {
 	switch fieldpath[0] {
 	// unhandled: timestamp
 	case "namespace":
-		return string(e.Namespace), len(e.Namespace) > 0
+		return e.Namespace, len(e.Namespace) > 0
 	case "topic":
-		return string(e.Topic), len(e.Topic) > 0
+		return e.Topic, len(e.Topic) > 0
 	case "event":
 		decoded, err := typeurl.UnmarshalAny(e.Event)
 		if err != nil {