[Foundation] String.Encoding: Modernize hashing

test/stdlib/NSStringAPI.swift

@@ -91,6 +91,17 @@ NSStringAPIs.test("NSStringEncoding") {
   expectEqual(.utf8, enc)
 }
 
+NSStringAPIs.test("NSStringEncoding.Hashable") {
+  let instances: [String.Encoding] = [
+    .windowsCP1250,
+    .utf32LittleEndian,
+    .utf32BigEndian,
+    .ascii,
+    .utf8,
+  ]
+  checkHashable(instances, equalityOracle: { $0 == $1 })
+}
+
 NSStringAPIs.test("localizedStringWithFormat(_:...)") {
   let world: NSString = "world"
   expectEqual("Hello, world!%42", String.localizedStringWithFormat(