fix cmake install directory (for real this time)

* Rename 'develop' folder to 'include/nlohmann'
* Rename 'src' folder to 'single_include/nlohmann'
* Use <nlohmann/*> headers in sources and tests
* Change amalgamate config file

include/nlohmann/detail/iterators/iteration_proxy.hpp

@@ -0,0 +1,98 @@
+#pragma once
+
+#include <cstddef> // size_t
+#include <string> // string, to_string
+
+#include <nlohmann/detail/value_t.hpp>
+
+namespace nlohmann
+{
+namespace detail
+{
+/// proxy class for the items() function
+template<typename IteratorType> class iteration_proxy
+{
+  private:
+    /// helper class for iteration
+    class iteration_proxy_internal
+    {
+      private:
+        /// the iterator
+        IteratorType anchor;
+        /// an index for arrays (used to create key names)
+        std::size_t array_index = 0;
+
+      public:
+        explicit iteration_proxy_internal(IteratorType it) noexcept : anchor(it) {}
+
+        /// dereference operator (needed for range-based for)
+        iteration_proxy_internal& operator*()
+        {
+            return *this;
+        }
+
+        /// increment operator (needed for range-based for)
+        iteration_proxy_internal& operator++()
+        {
+            ++anchor;
+            ++array_index;
+
+            return *this;
+        }
+
+        /// inequality operator (needed for range-based for)
+        bool operator!=(const iteration_proxy_internal& o) const noexcept
+        {
+            return anchor != o.anchor;
+        }
+
+        /// return key of the iterator
+        std::string key() const
+        {
+            assert(anchor.m_object != nullptr);
+
+            switch (anchor.m_object->type())
+            {
+                // use integer array index as key
+                case value_t::array:
+                    return std::to_string(array_index);
+
+                // use key from the object
+                case value_t::object:
+                    return anchor.key();
+
+                // use an empty key for all primitive types
+                default:
+                    return "";
+            }
+        }
+
+        /// return value of the iterator
+        typename IteratorType::reference value() const
+        {
+            return anchor.value();
+        }
+    };
+
+    /// the container to iterate
+    typename IteratorType::reference container;
+
+  public:
+    /// construct iteration proxy from a container
+    explicit iteration_proxy(typename IteratorType::reference cont) noexcept
+        : container(cont) {}
+
+    /// return iterator begin (needed for range-based for)
+    iteration_proxy_internal begin() noexcept
+    {
+        return iteration_proxy_internal(container.begin());
+    }
+
+    /// return iterator end (needed for range-based for)
+    iteration_proxy_internal end() noexcept
+    {
+        return iteration_proxy_internal(container.end());
+    }
+};
+}
+}