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/chainparams.h

@@ -8,7 +8,6 @@
 
 #include "bignum.h"
 #include "uint256.h"
-#include "util.h"
 
 #include <vector>