region: support default construction of step iterator

This allows use in some std::algorithms that assume the iterator
conforms to LegacyIterator

region.hpp

@@ -146,6 +146,12 @@ namespace cruft {
                 hi (_hi)
             { ; }
 
+            iterator ()
+                : cursor {}
+                , lo {}
+                , hi {}
+            { ; }
+
 
             const point_t& operator* (void) const& { return cursor; }