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/json_ref.hpp

@@ -0,0 +1,63 @@
+#pragma once
+
+#include <initializer_list>
+#include <utility>
+
+namespace nlohmann
+{
+namespace detail
+{
+template<typename BasicJsonType>
+class json_ref
+{
+  public:
+    using value_type = BasicJsonType;
+
+    json_ref(value_type&& value)
+        : owned_value(std::move(value)), value_ref(&owned_value), is_rvalue(true)
+    {}
+
+    json_ref(const value_type& value)
+        : value_ref(const_cast<value_type*>(&value)), is_rvalue(false)
+    {}
+
+    json_ref(std::initializer_list<json_ref> init)
+        : owned_value(init), value_ref(&owned_value), is_rvalue(true)
+    {}
+
+    template<class... Args>
+    json_ref(Args&& ... args)
+        : owned_value(std::forward<Args>(args)...), value_ref(&owned_value), is_rvalue(true)
+    {}
+
+    // class should be movable only
+    json_ref(json_ref&&) = default;
+    json_ref(const json_ref&) = delete;
+    json_ref& operator=(const json_ref&) = delete;
+
+    value_type moved_or_copied() const
+    {
+        if (is_rvalue)
+        {
+            return std::move(*value_ref);
+        }
+        return *value_ref;
+    }
+
+    value_type const& operator*() const
+    {
+        return *static_cast<value_type const*>(value_ref);
+    }
+
+    value_type const* operator->() const
+    {
+        return static_cast<value_type const*>(value_ref);
+    }
+
+  private:
+    mutable value_type owned_value = nullptr;
+    value_type* value_ref = nullptr;
+    const bool is_rvalue;
+};
+}
+}