factoring the specialized iterator adaptors

[SVN r19467]

doc/indirect_iterator_abstract.rst

@@ -0,0 +1,8 @@
+The indirect iterator adapts an iterator by applying an *extra*
+dereference inside of ``operator*()``. For example, this iterator
+adaptor makes it possible to view a container of pointers
+(e.g. ``list<foo*>``) as if it were a container of the pointed-to type
+(e.g. ``list<foo>``) .
+
+.. At some point we should add the capability to handle
+   iterators over smart pointers, which the impl handles. -JGS