cleanup: use range based for in download code

This removes a constant describing the length of the array.

The enumerated_range code had to be adapted, because the
interaction of C-type arrays with the C++ typesystem is mad.
With C-type arrays, one has to pass a reference to std::declval.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/range.h

@@ -40,7 +40,7 @@ class enumerated_range
 {
 	Range &base;
 public:
-	using base_iterator = decltype(std::begin(std::declval<Range>()));
+	using base_iterator = decltype(std::begin(std::declval<Range &>()));
 	class iterator {
 		int idx;
 		base_iterator it;