Add optimisation for the use of GSImmutableString so that it can make use of the layout of the underlying mutable string that the GSImmutableString is a proxy for.

Source/Additions/NSMutableString+GNUstepBase.m

@@ -34,10 +34,14 @@
 /* This private class is used for the -immutableProxy method in the category
  * on NSMutableString.
  * It is needed for [NSAttributedString-string] and [NSTextStorage-string]
+ *
+ * NB. The _parent instance variable is private but must *not* be changed
+ * because it is actually used by GSMutableString code to permit some
+ * optimisation (see GSString.m).
  */
 @interface	GSImmutableString : NSString
 {
-  NSString	*_parent;
+  NSString	*_parent;	// Do not change this ivar declaration
 }
 - (id) initWithString: (NSString*)parent;
 @end