Use NtCreateFile on Win32 (#27195)

NtCreateFile allows passing in a base handle which makes opening child directories much faster. This cuts anywhere from 3-10% of the time off of a GetFiles() call.

It is significantly faster as it avoids normalization and path parsing overhead.

Author: JeremyKuhne

src/Common/src/Interop/Windows/Interop.UNICODE_STRING.cs

@@ -7,10 +7,19 @@
 
 internal static partial class Interop
 {
+    // https://msdn.microsoft.com/en-us/library/windows/desktop/aa380518.aspx
+    // https://msdn.microsoft.com/en-us/library/windows/hardware/ff564879.aspx
     [StructLayout(LayoutKind.Sequential)]
     internal struct UNICODE_STRING
     {
+        /// <summary>
+        /// Length, in bytes, not including the the null, if any.
+        /// </summary>
         internal ushort Length;
+
+        /// <summary>
+        /// Max size of the buffer in bytes
+        /// </summary>
         internal ushort MaximumLength;
         internal IntPtr Buffer;
     }