Do not use implicit type conversion for address literals (#1002)

Uses the explicit address(0) for the zero address

contracts/Bounty.sol

@@ -40,7 +40,7 @@ contract Bounty is PullPayment, Destructible {
    */
   function claim(Target target) public {
     address researcher = researchers[target];
-    require(researcher != 0);
+    require(researcher != address(0));
     // Check Target contract invariants
     require(!target.checkInvariant());
     asyncSend(researcher, address(this).balance);

contracts/examples/SimpleSavingsWallet.sol

@@ -32,7 +32,7 @@ contract SimpleSavingsWallet is Heritable {
    * @dev wallet can send funds
    */
   function sendTo(address payee, uint256 amount) public onlyOwner {
-    require(payee != 0 && payee != address(this));
+    require(payee != address(0) && payee != address(this));
     require(amount > 0);
     payee.transfer(amount);
     emit Sent(payee, amount, address(this).balance);

contracts/examples/SimpleToken.sol

@@ -24,7 +24,7 @@ contract SimpleToken is StandardToken {
   constructor() public {
     totalSupply_ = INITIAL_SUPPLY;
     balances[msg.sender] = INITIAL_SUPPLY;
-    emit Transfer(0x0, msg.sender, INITIAL_SUPPLY);
+    emit Transfer(address(0), msg.sender, INITIAL_SUPPLY);
   }
 
 }

contracts/mocks/SafeERC20Helper.sol

@@ -70,26 +70,26 @@ contract SafeERC20Helper {
   }
 
   function doFailingTransfer() public {
-    failing.safeTransfer(0, 0);
+    failing.safeTransfer(address(0), 0);
   }
 
   function doFailingTransferFrom() public {
-    failing.safeTransferFrom(0, 0, 0);
+    failing.safeTransferFrom(address(0), address(0), 0);
   }
 
   function doFailingApprove() public {
-    failing.safeApprove(0, 0);
+    failing.safeApprove(address(0), 0);
   }
 
   function doSucceedingTransfer() public {
-    succeeding.safeTransfer(0, 0);
+    succeeding.safeTransfer(address(0), 0);
   }
 
   function doSucceedingTransferFrom() public {
-    succeeding.safeTransferFrom(0, 0, 0);
+    succeeding.safeTransferFrom(address(0), address(0), 0);
   }
 
   function doSucceedingApprove() public {
-    succeeding.safeApprove(0, 0);
+    succeeding.safeApprove(address(0), 0);
   }
 }

contracts/ownership/Heritable.sol

@@ -78,7 +78,7 @@ contract Heritable is Ownable {
    */
   function removeHeir() public onlyOwner {
     heartbeat();
-    heir_ = 0;
+    heir_ = address(0);
   }
 
   /**