Move helper headers into detail directory.

include/stream/streams/join.hpp

@@ -1,8 +1,9 @@
 #pragma once
 
-#include "self_iterating_container.hpp"
 #include <iterator>
 
+#include "detail/self_iterating_container.hpp"
+
 namespace stream { namespace detail {
   template <typename C> class join_iterator {
   public:

include/stream/streams/streams.hpp

@@ -5,7 +5,7 @@
 #include <numeric>
 #include <vector>
 
-#include "ifd_pointer.hpp"
+#include "detail/ifd_pointer.hpp"
 
 namespace stream {
   template <typename T> class iterator {