Remove usage of 5.2-only syntax.

Fixes #97.

Author: natecook1000

Sources/ArgumentParser/Parsing/ArgumentSet.swift

@@ -520,9 +520,11 @@ extension ArgumentSet {
     // Finished with the defined arguments; are there leftover values to parse?
     skipNonValues()
     guard argumentStack.isEmpty else {
-      let extraValues: [(InputOrigin, String)] = argumentStack.map(\.0).map {
-        return (InputOrigin(element: $0), unusedInput.originalInput(at: $0)!)
-      }
+      let extraValues: [(InputOrigin, String)] = argumentStack
+        .map { $0.0 }
+        .map {
+          (InputOrigin(element: $0), unusedInput.originalInput(at: $0)!)
+        }
       throw ParserError.unexpectedExtraValues(extraValues)
     }
   }