Trimmed down standard includes:

- no longer including <ostream>, using <iosfwd> instead
- under BOOST_LEAF_DIAGNOSTICS=0, no longer including <set> and <sstream>

test/preload_basic_test.cpp

@@ -7,6 +7,7 @@
 #include <boost/leaf/handle_errors.hpp>
 #include <boost/leaf/result.hpp>
 #include "lightweight_test.hpp"
+#include <sstream>
 
 namespace leaf = boost::leaf;