Use bytes8 index access (#19)

Co-authored-by: Alex Beregszaszi <[email protected]>

deposit_contract.sol

@@ -141,15 +141,16 @@ contract DepositContract is IDepositContract, ERC165 {
     }
 
     function to_little_endian_64(uint64 value) internal pure returns (bytes memory ret) {
-        // Unrolled the loop here.
         ret = new bytes(8);
-        ret[0] = bytes1(uint8(value));
-        ret[1] = bytes1(uint8(value >> 8));
-        ret[2] = bytes1(uint8(value >> 16));
-        ret[3] = bytes1(uint8(value >> 24));
-        ret[4] = bytes1(uint8(value >> 32));
-        ret[5] = bytes1(uint8(value >> 40));
-        ret[6] = bytes1(uint8(value >> 48));
-        ret[7] = bytes1(uint8(value >> 56));
+        bytes8 bytesValue = bytes8(value);
+        // Byteswapping during copying to bytes.
+        ret[0] = bytesValue[7];
+        ret[1] = bytesValue[6];
+        ret[2] = bytesValue[5];
+        ret[3] = bytesValue[4];
+        ret[4] = bytesValue[3];
+        ret[5] = bytesValue[2];
+        ret[6] = bytesValue[1];
+        ret[7] = bytesValue[0];
     }
 }