Add string_stream pipe modifiers

include/mtl/stream.cpp

@@ -0,0 +1,14 @@
+#include "mtl/stream.hpp"
+
+namespace mtl {
+
+_endl endl;
+_flush flush;
+
+_dec dec;
+_hex hex;
+
+_left left;
+_right right;
+
+} // namespace mtl

include/mtl/stream.hpp

@@ -0,0 +1,20 @@
+#pragma once
+
+#include <cstddef>
+
+namespace mtl {
+
+extern struct _endl { } endl;
+extern struct _flush { } flush;
+
+extern struct _dec { } dec;
+extern struct _hex { } hex;
+
+extern struct _left { } left;
+extern struct _right { } right;
+struct setw {
+	size_t m_w;
+	setw(size_t w) : m_w(w) { }
+};
+
+} // namespace mtl