Cleanup code using forward declarations.

Use misc methods of avoiding unnecesary header includes.
Replace int typedefs with int##_t from stdint.h.
Replace PRI64[xdu] with PRI[xdu]64 from inttypes.h.
Normalize QT_VERSION ifs where possible.
Resolve some indirect dependencies as direct ones.
Remove extern declarations from .cpp files.

src/test/Checkpoints_tests.cpp

@@ -1,12 +1,12 @@
 //
 // Unit tests for block-chain checkpoints
 //
-#include <boost/assign/list_of.hpp> // for 'map_list_of()'
-#include <boost/test/unit_test.hpp>
-#include <boost/foreach.hpp>
 
-#include "../checkpoints.h"
-#include "../util.h"
+#include "checkpoints.h"
+
+#include "uint256.h"
+
+#include <boost/test/unit_test.hpp>
 
 using namespace std;