irange: introduce one-parameter variant of irange

this simplifies the use of boost::irange, as boost::irange( 0, foo );
may compile or not depending on the platform-specific integer type of
foo, requiring an explicit cast of 0 to decltype(foo).

doc/reference/ranges/irange.qbk

@@ -10,7 +10,11 @@
 ``
 template<class Integer>
 iterator_range< range_detail::integer_iterator<Integer> >
-irange(Integer first, Integer  last);
+irange(Integer last);
+
+template<class Integer>
+iterator_range< range_detail::integer_iterator<Integer> >
+irange(Integer first, Integer last);
 
 template<class Integer, class StepSize>
 iterator_range< range_detail::integer_iterator_with_step<Integer, StepSize> >
@@ -37,4 +41,3 @@ Defined in the header file `boost/range/irange.hpp`
 Constant. Since this function generates a new range the most significant performance cost is incurred through the iteration of the generated range.
 
 [endsect]
-