Migrate contracts to Solidity 0.7 (#2319)

* Update contract pragmas to solidity 0.7

* Remove internal declaration on constructors

* Reference SafeMath explicitely

* Remove public constructor declaration from abstract contracts

* Remove public constructor declaration from non-abstract contracts

contracts/mocks/PullPaymentMock.sol

@@ -1,12 +1,12 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 import "../payment/PullPayment.sol";
 
 // mock class using PullPayment
 contract PullPaymentMock is PullPayment {
-    constructor () public payable { }
+    constructor () payable { }
 
     // test helper function to call asyncTransfer
     function callTransfer(address dest, uint256 amount) public {