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/bignum.h

@@ -222,7 +222,7 @@ public:
         BN_mpi2bn(pch, p - pch, this);
     }
 
-    uint256 getuint256()
+    uint256 getuint256() const
     {
         unsigned int nSize = BN_bn2mpi(this, NULL);
         if (nSize < 4)