fix: use SafeMath more consistently

Author: pcarranzav

contracts/gateway/L1GraphTokenGateway.sol

@@ -212,7 +212,7 @@ contract L1GraphTokenGateway is GraphTokenGateway, L1ArbitrumMessenger {
                     // makes sure only sufficient ETH is supplied as required for successful redemption on L2
                     // if a user does not desire immediate redemption they should provide
                     // a msg.value of AT LEAST maxSubmissionCost
-                    uint256 expectedEth = maxSubmissionCost + (_maxGas * _gasPriceBid);
+                    uint256 expectedEth = maxSubmissionCost.add(_maxGas.mul(_gasPriceBid));
                     require(msg.value >= expectedEth, "WRONG_ETH_VALUE");
                 }
                 outboundCalldata = getOutboundCalldata(_l1Token, from, _to, _amount, extraData);

contracts/reservoir/L1Reservoir.sol

@@ -298,14 +298,14 @@ contract L1Reservoir is L1ReservoirV1Storage, Reservoir {
      * @param _globalDelta New global rewards (i.e. rewards on L1 and L2) since the last update block
      */
     function snapshotAccumulatedRewards(uint256 _globalDelta) internal {
-        issuanceBase = issuanceBase + _globalDelta;
+        issuanceBase = issuanceBase.add(_globalDelta);
         // Reimplementation of getAccumulatedRewards but reusing the _globalDelta calculated above,
         // to save gas
-        accumulatedLayerRewards =
-            accumulatedLayerRewards +
+        accumulatedLayerRewards = accumulatedLayerRewards.add(
             _globalDelta.mul(FIXED_POINT_SCALING_FACTOR.sub(l2RewardsFraction)).div(
                 FIXED_POINT_SCALING_FACTOR
-            );
+            )
+        );
         lastRewardsUpdateBlock = block.number;
     }
 

contracts/reservoir/Reservoir.sol

@@ -36,7 +36,7 @@ abstract contract Reservoir is GraphUpgradeable, ReservoirV1Storage, IReservoir
      */
     function getAccumulatedRewards(uint256 _blocknum) public view override returns (uint256) {
         // R(t) = R(t0) + (DeltaR(t, t0))
-        return accumulatedLayerRewards + getNewRewards(_blocknum);
+        return accumulatedLayerRewards.add(getNewRewards(_blocknum));
     }
 
     /**