Enable SwiftLint rule 'implicit_return' and fix violation

.swiftlint.yml

@@ -79,7 +79,7 @@ whitelist_rules:
   - identical_operands
 #  - identifier_name
   - implicit_getter
-#  - implicit_return
+  - implicit_return
 #  - implicitly_unwrapped_optional
 #  - indentation_width
   - inert_defer

passKit/Parser/AdditionField.swift

@@ -19,7 +19,7 @@ public struct AdditionField: Hashable {
     }
 
     var asTuple: (String, String) {
-        return (title, content)
+        (title, content)
     }
 }