Fix constness on dereferencing a concat_iteratort

Dereferencing a `concat_iteratort` from concating const collections
with `ranget` would previously introduce a compile error, due to a
`const` to non-const conversion. This commit fixes this, by using the
same return type as the iterator which it wraps around.

This commit includes a test for using range to concat `const`
collections.

Author: thomasspriggs

src/util/range.h

@@ -208,8 +208,8 @@ struct concat_iteratort
 public:
   using difference_type = typename first_iteratort::difference_type;
   using value_type = typename first_iteratort::value_type;
-  using pointer = const value_type *;
-  using reference = const value_type &;
+  using pointer = typename first_iteratort::pointer;
+  using reference = typename first_iteratort::reference;
   using iterator_category = std::forward_iterator_tag;
 
   static_assert(
@@ -245,14 +245,14 @@ struct concat_iteratort
     return tmp;
   }
 
-  value_type &operator*()
+  reference operator*()
   {
     if(first_begin == first_end)
       return *second_begin;
     return *first_begin;
   }
 
-  value_type *operator->()
+  pointer operator->()
   {
     if(first_begin == first_end)
       return &(*second_begin);

unit/util/range.cpp

@@ -76,6 +76,18 @@ SCENARIO("range tests", "[core][util][range]")
       REQUIRE(odds == expected_odds);
     }
   }
+  GIVEN("Two const vectors of ints")
+  {
+    const std::vector<int> input1{1, 2};
+    const std::vector<int> input2{3, 4};
+    THEN("Concat the vectors using range.")
+    {
+      auto range = make_range(input1).concat(make_range(input2));
+      const std::vector<int> output{range.begin(), range.end()};
+      const std::vector<int> expected{1, 2, 3, 4};
+      REQUIRE(output == expected);
+    };
+  }
 }
 
 class move_onlyt