Use 0.8.0 for echidna exercises

Author: montyly

.github/workflows/echidna.yml

@@ -167,7 +167,7 @@ jobs:
           contract: ${{ matrix.contract }}
           config: ${{ matrix.config }}
           output-file: ${{ matrix.files }}.out
-          solc-version: ${{ matrix.solc-version || '0.5.11' }}
+          solc-version: ${{ matrix.solc-version || '0.8.0' }}
           echidna-workdir: ${{ matrix.workdir }}
           echidna-version: edge
           crytic-args: ${{ matrix.crytic-args || '' }}

program-analysis/echidna/exercises/Exercise-1.md

@@ -15,7 +15,7 @@ Join the team on Slack at: https://empireslacking.herokuapp.com/ #ethereum
 We will test the following contract _[token.sol](https://github.com/crytic/building-secure-contracts/tree/master/program-analysis/echidna/exercises/exercise1/token.sol)_:
 
 ```solidity
-pragma solidity ^0.5.0;
+pragma solidity ^0.8.0;
 
 contract Ownable {
     address public owner = msg.sender;
@@ -67,13 +67,13 @@ contract Token is Ownable, Pausable {
 The skeleton for this exercise is (_[template.sol](https://github.com/crytic/building-secure-contracts/tree/master/program-analysis/echidna/exercises/exercise1/template.sol)_):
 
 ````solidity
-pragma solidity ^0.5.0;
+pragma solidity ^0.8.0;
 
 import "./token.sol";
 
 /// @dev Run the template with
 ///      ```
-///      solc-select use 0.5.0
+///      solc-select use 0.8.0
 ///      echidna program-analysis/echidna/exercises/exercise1/template.sol
 ///      ```
 contract TestToken is Token {

program-analysis/echidna/exercises/Exercise-2.md

@@ -17,7 +17,7 @@ Join the team on Slack at: https://empireslacking.herokuapp.com/ #ethereum
 We will test the following contract _[token.sol](https://github.com/crytic/building-secure-contracts/tree/master/program-analysis/echidna/exercises/exercise2/token.sol)_:
 
 ```solidity
-pragma solidity ^0.5.0;
+pragma solidity ^0.8.0;
 
 contract Ownable {
     address public owner = msg.sender;
@@ -74,7 +74,7 @@ contract Token is Ownable, Pausable {
 The skeleton for this exercise is (_[template.sol](https://github.com/crytic/building-secure-contracts/tree/master/program-analysis/echidna/exercises/exercise2/template.sol)_):
 
 ````solidity
-pragma solidity ^0.5.0;
+pragma solidity ^0.8.0;
 
 import "./token.sol";
 

program-analysis/echidna/exercises/Exercise-3.md

@@ -17,7 +17,7 @@ Join the team on Slack at: https://empireslacking.herokuapp.com/ #ethereum
 We will test the following contract _[token.sol](https://github.com/crytic/building-secure-contracts/tree/master/program-analysis/echidna/exercises/exercise3/token.sol)_:
 
 ```solidity
-pragma solidity ^0.5.0;
+pragma solidity ^0.8.0;
 
 /// @notice The issues from exercise 1 and 2 are fixed.
 
@@ -66,7 +66,7 @@ contract Token is Ownable, Pausable {
 Consider the following extension of the token (_[mintable.sol](https://github.com/crytic/building-secure-contracts/tree/master/program-analysis/echidna/exercises/exercise3/mintable.sol)_):
 
 ```solidity
-pragma solidity ^0.5.0;
+pragma solidity ^0.8.0;
 
 import "./token.sol";
 
@@ -98,7 +98,7 @@ The [version of token.sol](https://github.com/crytic/building-secure-contracts/t
 The skeleton for this exercise is (_[template.sol](https://github.com/crytic/building-secure-contracts/tree/master/program-analysis/echidna/exercises/exercise3/template.sol)_):
 
 ````solidity
-pragma solidity ^0.5.0;
+pragma solidity ^0.8.0;
 
 import "./mintable.sol";
 

program-analysis/echidna/exercises/Exercise-4.md

@@ -17,7 +17,7 @@ Join the team on Slack at: https://empireslacking.herokuapp.com/ #ethereum
 We will test the following contract _[token.sol](https://github.com/crytic/building-secure-contracts/tree/master/program-analysis/echidna/exercises/exercise4/token.sol)_:
 
 ```solidity
-pragma solidity ^0.5.0;
+pragma solidity ^0.8.0;
 
 contract Ownable {
     address public owner = msg.sender;
@@ -79,13 +79,13 @@ This exercise is similar to the [first one](Exercise-1.md), but using assertions
 The skeleton for this exercise is (_[template.sol](https://github.com/crytic/building-secure-contracts/tree/master/program-analysis/echidna/exercises/exercise4/template.sol)_):
 
 ````solidity
-pragma solidity ^0.5.0;
+pragma solidity ^0.8.0;
 
 import "./token.sol";
 
 /// @dev Run the template with
 ///      ```
-///      solc-select use 0.5.0
+///      solc-select use 0.8.0
 ///      echidna program-analysis/echidna/exercises/exercise4/template.sol --contract TestToken --test-mode assertion
 ///      ```
 contract TestToken is Token {

program-analysis/echidna/exercises/exercise1/solution.sol

@@ -1,11 +1,11 @@
 // SPDX-License-Identifier: AGPL-3.0
-pragma solidity ^0.5.0;
+pragma solidity ^0.8.0;
 
 import "./token.sol";
 
 /// @dev Run the solution with
 ///      ```
-///      solc-select use 0.5.0
+///      solc-select use 0.8.0
 ///      echidna program-analysis/echidna/exercises/exercise1/solution.sol
 ///      ```
 contract TestToken is Token {

program-analysis/echidna/exercises/exercise1/template.sol

@@ -1,11 +1,11 @@
 // SPDX-License-Identifier: AGPL-3.0
-pragma solidity ^0.5.0;
+pragma solidity ^0.8.0;
 
 import "./token.sol";
 
 /// @dev Run the template with
 ///      ```
-///      solc-select use 0.5.0
+///      solc-select use 0.8.0
 ///      echidna program-analysis/echidna/exercises/exercise1/template.sol
 ///      ```
 contract TestToken is Token {

program-analysis/echidna/exercises/exercise1/token.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: AGPL-3.0
-pragma solidity ^0.5.0;
+pragma solidity ^0.8.0;
 
 contract Ownable {
     address public owner = msg.sender;
@@ -35,7 +35,10 @@ contract Token is Ownable, Pausable {
     mapping(address => uint256) public balances;
 
     function transfer(address to, uint256 value) public whenNotPaused {
-        balances[msg.sender] -= value;
-        balances[to] += value;
+        // unchecked to save gas
+        unchecked{
+            balances[msg.sender] -= value;
+            balances[to] += value;
+        }
     }
 }

program-analysis/echidna/exercises/exercise2/solution.sol

@@ -1,11 +1,11 @@
 // SPDX-License-Identifier: AGPL-3.0
-pragma solidity ^0.5.0;
+pragma solidity ^0.8.0;
 
 import "./token.sol";
 
 /// @dev Run the solution with
 ///      ```
-///      solc-select use 0.5.0
+///      solc-select use 0.8.0
 ///      echidna program-analysis/echidna/exercises/exercise1/solution.sol
 ///      ```
 contract TestToken is Token {

program-analysis/echidna/exercises/exercise2/template.sol

@@ -1,11 +1,11 @@
 // SPDX-License-Identifier: AGPL-3.0
-pragma solidity ^0.5.0;
+pragma solidity ^0.8.0;
 
 import "./token.sol";
 
 /// @dev Run the template with
 ///      ```
-///      solc-select use 0.5.0
+///      solc-select use 0.8.0
 ///      echidna program-analysis/echidna/exercises/exercise2/template.sol
 ///      ```
 contract TestToken is Token {