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/utils/Address.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.6.2;
+pragma solidity ^0.7.0;
 
 /**
  * @dev Collection of functions related to the address type

contracts/utils/Arrays.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 import "../math/Math.sol";
 

contracts/utils/Counters.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 import "../math/SafeMath.sol";
 

contracts/utils/Create2.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 /**
  * @dev Helper to make usage of the `CREATE2` EVM opcode easier and safer.

contracts/utils/EnumerableMap.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 /**
  * @dev Library for managing an enumerable variant of Solidity's

contracts/utils/EnumerableSet.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 /**
  * @dev Library for managing

contracts/utils/Pausable.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 import "../GSN/Context.sol";
 
@@ -29,7 +29,7 @@ contract Pausable is Context {
     /**
      * @dev Initializes the contract in unpaused state.
      */
-    constructor () internal {
+    constructor () {
         _paused = false;
     }
 

contracts/utils/ReentrancyGuard.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 /**
  * @dev Contract module that helps prevent reentrant calls to a function.
@@ -35,7 +35,7 @@ contract ReentrancyGuard {
 
     uint256 private _status;
 
-    constructor () internal {
+    constructor () {
         _status = _NOT_ENTERED;
     }
 

contracts/utils/SafeCast.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 
 /**

contracts/utils/Strings.sol

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity ^0.6.0;
+pragma solidity ^0.7.0;
 
 /**
  * @dev String operations.