Use a uint256 for bnChainWork

Every block index entry currently requires a separately-allocated
CBigNum. By replacing them with uint256, it's just 32 bytes extra
in CBlockIndex itself.

This should save us a few megabytes in RAM, and less allocation
overhead.

src/uint256.h

@@ -55,6 +55,16 @@ public:
         return ret;
     }
 
+    double getdouble() const
+    {
+        double ret = 0.0;
+        double fact = 1.0;
+        for (int i = 0; i < WIDTH; i++) {
+            ret += fact * pn[i];
+            fact *= 4294967296.0;
+        }
+        return ret;
+    }
 
     base_uint& operator=(uint64 b)
     {