safe unordered local iterators

[Paolo Carlini]

Hi,
> Ok to apply ?
I think the patch can go in, but before that, please fix some coding 
convention issues:
1- I spotted quite a few overlong lines.
2- In simple conditionals avoid redundant curly brackets, eg

+    if (_M_sequence)
+      {
+	_M_get_sequence()->_M_detach_local(this);
+      }

or

+	    if (!__victim->_M_singular()&&  __pred(__victim->base()))
+	      {
+		__victim->_M_invalidate();
+	      }

Many of those.
3- I also see a misplaced bracket:

+	switch (__dist.second) {

4- Finally, not a big deal, but I think you want the __get_distance 
inline, are all very simple (on the other hand, at some point we should 
go through the entire debug/ directory and move out of line many large 
member functions currently inline).

Thanks!
Paolo.