implement collateral deposit/redemption accounting in ZToken

Author: ademolahh

src/ZToken.sol

@@ -148,13 +148,25 @@ abstract contract ZToken is IZToken, ERC20 {
     function _depositCollateral(uint256 _underlyingAmount) internal {
         if (_underlyingAmount == 0) revert ZeroAmountIsNotAllowed();
 
-        //
+        _transferIn(_underlyingAmount);
+
+        uint256 newCollateralBalance = collateralAmount[msg.sender] + _underlyingAmount;
+        collateralAmount[msg.sender] = newCollateralBalance;
 
-        emit CollateralDeposited(msg.sender, _underlyingAmount, _underlyingAmount);
+        emit CollateralDeposited(msg.sender, _underlyingAmount, newCollateralBalance);
     }
 
     function _redeemForCollateral(uint256 _zAmount) internal {
         if (_zAmount == 0) revert ZeroAmountIsNotAllowed();
+
+        uint256 underlyingAmount = _zAmount.mulWad(getExchangeRate());
+
+        _burn(msg.sender, _zAmount);
+
+        uint256 newCollateralBalance = collateralAmount[msg.sender] + underlyingAmount;
+        collateralAmount[msg.sender] = newCollateralBalance;
+
+        emit CollateralRedeemed(msg.sender, _zAmount, underlyingAmount, newCollateralBalance);
     }
 
     function _transferIn(uint256 _amount) internal virtual { }