ZipFile: Reduce allocations in IsDirEmpty

- Use Directory.EnumerateFileSystemEntries to avoid the extra
allocations associated with the FileInfo/DirectoryInfo instance
created by DirectoryInfo.EnumerateFileSystemInfos.
- Use the default overload of EnumerateFileSystemEntries, which uses
SearchOption.TopDirectoryOnly, as SearchOption.AllDirectories is not
necessary here and AllDirectories currently has more allocations than
TopDirectoryOnly on Windows.
- Use the enumerator directly to avoid the unnecessary call to
Current.

Author: justinvp

src/System.IO.Compression.ZipFile/src/System/IO/Compression/ZipFile.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
+using System.Collections.Generic;
 using System.Diagnostics;
 using System.Text;
 
@@ -569,10 +570,8 @@ private static string EntryFromPath(string entry, int offset, int length)
 
         private static Boolean IsDirEmpty(DirectoryInfo possiblyEmptyDir)
         {
-            foreach (FileSystemInfo fi in possiblyEmptyDir.EnumerateFileSystemInfos("*", SearchOption.AllDirectories))
-                return false;
-
-            return true;
+            using (IEnumerator<String> enumerator = Directory.EnumerateFileSystemEntries(possiblyEmptyDir.FullName).GetEnumerator())
+                return !enumerator.MoveNext();
         }
     }  // class ZipFile
 }  // namespace