change zone type name to zonename

This should make it more obvious for what this type is intended.

types/zone.go

@@ -7,15 +7,15 @@ import (
 
 type (
 	// Zone represents the name of a zone. A zone must end with a `.`.
-	Zone string
+	ZoneName string
 )
 
-func (z *Zone) UnmarshalJSON(in []byte) error {
+func (z *ZoneName) UnmarshalJSON(in []byte) error {
 	in = bytes.Trim(in, `"`)
 
 	if in[len(in)-1] != '.' {
 		return fmt.Errorf("not a valid zone name")
 	}
-	*z = Zone(in)
+	*z = ZoneName(in)
 	return nil
 }