indentlog: refactor: move stream-inserters to print/ subdir

include/indentlog/concat.hpp

@@ -1,37 +0,0 @@
-/* @file concat.hpp */
-
-#pragma once
-
-#include <ostream>
-#include <utility>  // for std::move()
-
-namespace xo {
-    template <typename T1, typename T2>
-    struct concat_impl {
-    public:
-        concat_impl(T1 && x1, T2 && x2)
-            : x1_{std::forward<T1>(x1)}, x2_{std::forward<T2>(x2)} {}
-
-        T1 const & x1() const { return x1_; }
-        T2 const & x2() const { return x2_; }
-
-    private:
-        T1 x1_;
-        T2 x2_;
-    }; /*concat_impl*/
-
-    template <typename T1, typename T2>
-    concat_impl<T1,T2> concat(T1 && x1, T2 && x2) {
-        return concat_impl<T1,T2>(std::move(x1), std::move(x2));
-    } /*concat*/
-
-    template <typename T1, typename T2>
-    inline std::ostream &
-    operator<<(std::ostream & os, concat_impl<T1, T2> const & x) {
-        os << x.x1() << x.x2();
-        return os;
-    } /*operator<<*/
-
-} /*namespace xo*/
-
-/* end concat.hpp */