Fix loop index var types, fixing many minor sign comparison warnings

foo.size() typically returns an unsigned integral type; make loop variables
match those types' signedness.

src/uint256.h

@@ -287,7 +287,7 @@ public:
     std::string GetHex() const
     {
         char psz[sizeof(pn)*2 + 1];
-        for (int i = 0; i < sizeof(pn); i++)
+        for (unsigned int i = 0; i < sizeof(pn); i++)
             sprintf(psz + i*2, "%02x", ((unsigned char*)pn)[sizeof(pn) - i - 1]);
         return std::string(psz, psz + sizeof(pn)*2);
     }