fix spurious include

...which causes problems when a preceding include
has already dragged in <functional>

the actual problem is the std::hash hack, which probably
is even no longer possible and could be removed (but
I don't have the time to investigate this somewhat
tricky topic right now)

To prevent this confusing situation, I'm adding the
include of "lib/symbol.hpp", to ensure we do have
the actual definitions of string and Literal,
which trait.hpp just declares forward.

An note, lib/symbol.hpp also includes hash-standard.hpp
first, so we avoid triggering problematic situation
from a header (format-cout.hpp), which is pervasively used
all over the place....

src/lib/format-obj.hpp

@@ -46,7 +46,7 @@
 #ifndef LIB_FORMAT_OBJ_H
 #define LIB_FORMAT_OBJ_H
 
-#include <string>
+#include "lib/symbol.hpp"
 #include "lib/meta/trait.hpp"
 
 #include <boost/lexical_cast.hpp>

src/lib/format-util.hpp

@@ -38,7 +38,6 @@
 #ifndef LIB_FORMAT_UTIL_H
 #define LIB_FORMAT_UTIL_H
 
-#include "lib/hash-standard.hpp"
 #include "lib/meta/trait.hpp"
 #include "lib/format-obj.hpp"
 #include "lib/itertools.hpp"