Add ERC20 compatibility to ERC777. (#1735)

* Add ERC20 compatibility.

* Reusing ERC20 tests for ERC777.

* Improve documentation.

* Add changelog entry.

* Improved ERC20 behavior tests.

* Add revert reasons to ERC777.

* ERC20 methods allow sending tokens to contracts with no interface.

* Register ERC20 interface.

* Add comment about avoidLockingTokens.

* Improve revert reason string.

* Make ERC777 implement IERC20.

* Fix test revert string.

* Remove unnecesary require.

* Add private _transfer.

* Update contracts/drafts/ERC777/ERC777.sol

Co-Authored-By: nventuro <nicolas.venturo@gmail.com>

* Update private helper names.

contracts/token/ERC20/ERC20.sol

@@ -20,7 +20,7 @@ contract ERC20 is IERC20 {
 
     mapping (address => uint256) private _balances;
 
-    mapping (address => mapping (address => uint256)) private _allowed;
+    mapping (address => mapping (address => uint256)) private _allowances;
 
     uint256 private _totalSupply;
 
@@ -47,7 +47,7 @@ contract ERC20 is IERC20 {
      * @return A uint256 specifying the amount of tokens still available for the spender.
      */
     function allowance(address owner, address spender) public view returns (uint256) {
-        return _allowed[owner][spender];
+        return _allowances[owner][spender];
     }
 
     /**
@@ -84,13 +84,13 @@ contract ERC20 is IERC20 {
      */
     function transferFrom(address from, address to, uint256 value) public returns (bool) {
         _transfer(from, to, value);
-        _approve(from, msg.sender, _allowed[from][msg.sender].sub(value));
+        _approve(from, msg.sender, _allowances[from][msg.sender].sub(value));
         return true;
     }
 
     /**
      * @dev Increase the amount of tokens that an owner allowed to a spender.
-     * approve should be called when _allowed[msg.sender][spender] == 0. To increment
+     * approve should be called when _allowances[msg.sender][spender] == 0. To increment
      * allowed value is better to use this function to avoid 2 calls (and wait until
      * the first transaction is mined)
      * From MonolithDAO Token.sol
@@ -99,13 +99,13 @@ contract ERC20 is IERC20 {
      * @param addedValue The amount of tokens to increase the allowance by.
      */
     function increaseAllowance(address spender, uint256 addedValue) public returns (bool) {
-        _approve(msg.sender, spender, _allowed[msg.sender][spender].add(addedValue));
+        _approve(msg.sender, spender, _allowances[msg.sender][spender].add(addedValue));
         return true;
     }
 
     /**
      * @dev Decrease the amount of tokens that an owner allowed to a spender.
-     * approve should be called when _allowed[msg.sender][spender] == 0. To decrement
+     * approve should be called when _allowances[msg.sender][spender] == 0. To decrement
      * allowed value is better to use this function to avoid 2 calls (and wait until
      * the first transaction is mined)
      * From MonolithDAO Token.sol
@@ -114,7 +114,7 @@ contract ERC20 is IERC20 {
      * @param subtractedValue The amount of tokens to decrease the allowance by.
      */
     function decreaseAllowance(address spender, uint256 subtractedValue) public returns (bool) {
-        _approve(msg.sender, spender, _allowed[msg.sender][spender].sub(subtractedValue));
+        _approve(msg.sender, spender, _allowances[msg.sender][spender].sub(subtractedValue));
         return true;
     }
 
@@ -171,7 +171,7 @@ contract ERC20 is IERC20 {
         require(owner != address(0), "ERC20: approve from the zero address");
         require(spender != address(0), "ERC20: approve to the zero address");
 
-        _allowed[owner][spender] = value;
+        _allowances[owner][spender] = value;
         emit Approval(owner, spender, value);
     }
 
@@ -185,6 +185,6 @@ contract ERC20 is IERC20 {
      */
     function _burnFrom(address account, uint256 value) internal {
         _burn(account, value);
-        _approve(account, msg.sender, _allowed[account][msg.sender].sub(value));
+        _approve(account, msg.sender, _allowances[account][msg.sender].sub(value));
     }
 }