[src] Change some NSString API to take/return NativeHandle instead of IntPtr for .NET. (#13492)

Co-authored-by: Manuel de la Pena <mandel@microsoft.com>

src/Foundation/NSString.cs

@@ -67,7 +67,7 @@ namespace Foundation {
 		{
 		}
 
-		static IntPtr CreateWithCharacters (IntPtr handle, string str, int offset, int length, bool autorelease = false)
+		static NativeHandle CreateWithCharacters (NativeHandle handle, string str, int offset, int length, bool autorelease = false)
 		{
 			unsafe {
 				fixed (char *ptrFirstChar = str) {
@@ -87,28 +87,28 @@ namespace Foundation {
 		}
 
 		[Obsolete ("Use of 'CFString.CreateNative' offers better performance.")]
-		public static IntPtr CreateNative (string str)
+		public static NativeHandle CreateNative (string str)
 		{
 			return CreateNative (str, false);
 		}
 
-		public static IntPtr CreateNative (string str, bool autorelease)
+		public static NativeHandle CreateNative (string str, bool autorelease)
 		{
-			if (str == null)
-				return IntPtr.Zero;
+			if (str is null)
+				return NativeHandle.Zero;
 
 			return CreateNative (str, 0, str.Length, autorelease);
 		}
 
-		public static IntPtr CreateNative (string value, int start, int length)
+		public static NativeHandle CreateNative (string value, int start, int length)
 		{
 			return CreateNative (value, start, length, false);
 		}
 
-		public static IntPtr CreateNative (string value, int start, int length, bool autorelease)
+		public static NativeHandle CreateNative (string value, int start, int length, bool autorelease)
 		{
-			if (value == null)
-				return IntPtr.Zero;
+			if (value is null)
+				return NativeHandle.Zero;
 
 			if (start < 0 || start > value.Length)
 				throw new ArgumentOutOfRangeException (nameof (start));
@@ -125,7 +125,7 @@ namespace Foundation {
 			return CreateWithCharacters (handle, value, start, length, autorelease);
 		}
 
-		public static void ReleaseNative (IntPtr handle)
+		public static void ReleaseNative (NativeHandle handle)
 		{
 			NSObject.DangerousRelease (handle);
 		}
@@ -170,15 +170,15 @@ namespace Foundation {
 		}
 
 		[Obsolete ("Use of 'CFString.FromHandle' offers better performance.")]
-		public static string FromHandle (IntPtr usrhandle)
+		public static string FromHandle (NativeHandle usrhandle)
 		{
 			return FromHandle (usrhandle, false);
 		}
 
 		[Obsolete ("Use of 'CFString.FromHandle' offers better performance.")]
-		public static string FromHandle (IntPtr handle, bool owns)
+		public static string FromHandle (NativeHandle handle, bool owns)
 		{
-			if (handle == IntPtr.Zero)
+			if (handle == NativeHandle.Zero)
 				return null;
 
 			try {