Reduce allocations in Win32 file system enumeration

- Lazily create and cache SearchResultHandler<TSource> instances
- Only allocate _searchStack when needed
- Remove the unnecessary SearchOption field on SearchData
- Replace SearchData class with unified PathPair struct with Unix.
- Remove the SearchResult class
- Pass WIN32_FIND_DATA by ref instead of copying (large struct)
- Various minor cleanup

Author: justinvp

src/System.IO.FileSystem/src/System.IO.FileSystem.csproj

@@ -43,6 +43,7 @@
     <Compile Include="System\IO\IFileSystemObject.cs" />
     <Compile Include="System\IO\Iterator.cs" />
     <Compile Include="System\IO\PathHelpers.cs" />
+    <Compile Include="System\IO\PathPair.cs" />
     <Compile Include="System\IO\ReadLinesIterator.cs" />
     <Compile Include="System\IO\SearchOption.cs" />
     <Compile Include="System\IO\SearchTarget.cs" />

src/System.IO.FileSystem/src/System/IO/PathPair.cs

@@ -0,0 +1,17 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+namespace System.IO
+{
+    internal struct PathPair
+    {
+        internal readonly string UserPath;
+        internal readonly string FullPath;
+
+        internal PathPair(string userPath, string fullPath)
+        {
+            UserPath = userPath;
+            FullPath = fullPath;
+        }
+    }
+}

src/System.IO.FileSystem/src/System/IO/UnixFileSystem.cs

@@ -524,18 +524,6 @@ private IEnumerator<T> Enumerate(Interop.libc.SafeDirHandle dirHandle)
                 }
             }
 
-            private struct PathPair
-            {
-                internal readonly string UserPath;
-                internal readonly string FullPath;
-
-                internal PathPair(string userPath, string fullPath)
-                {
-                    UserPath = userPath;
-                    FullPath = fullPath;
-                }
-            }
-
             private static string NormalizeSearchPattern(string searchPattern)
             {
                 if (searchPattern == "." || searchPattern == "*.*")