Replace use of BEGIN and END macros on uint256

Replace use of `BEGIN` and `END` macros on uint256 with `begin()` and
`end()` methods in the Merkle tree code.

src/merkleblock.cpp

@@ -53,7 +53,7 @@ uint256 CPartialMerkleTree::CalcHash(int height, unsigned int pos, const std::ve
         else
             right = left;
         // combine subhashes
-        return Hash(BEGIN(left), END(left), BEGIN(right), END(right));
+        return Hash(left.begin(), left.end(), right.begin(), right.end());
     }
 }
 
@@ -109,7 +109,7 @@ uint256 CPartialMerkleTree::TraverseAndExtract(int height, unsigned int pos, uns
             right = left;
         }
         // and combine them before returning
-        return Hash(BEGIN(left), END(left), BEGIN(right), END(right));
+        return Hash(left.begin(), left.end(), right.begin(), right.end());
     }
 }