Fixing withdrawal functionality in stBTC (#350)

Upon withdrawal we need to take into account fees that will be collected
by the treasury. A user will get the desired assets, but on top of that
we need to pull a bit more funds from Mezo covering withdrawal fees.

Added tests to cover scenarios with allocated funds to Mezo Portal and
then withdrawing and redeeming from Acre.

solidity/contracts/lib/ERC4626Fees.sol

@@ -103,7 +103,7 @@ abstract contract ERC4626Fees is ERC4626Upgradeable {
     function _feeOnRaw(
         uint256 assets,
         uint256 feeBasisPoints
-    ) private pure returns (uint256) {
+    ) internal pure returns (uint256) {
         return
             assets.mulDiv(
                 feeBasisPoints,
@@ -117,7 +117,7 @@ abstract contract ERC4626Fees is ERC4626Upgradeable {
     function _feeOnTotal(
         uint256 assets,
         uint256 feeBasisPoints
-    ) private pure returns (uint256) {
+    ) internal pure returns (uint256) {
         return
             assets.mulDiv(
                 feeBasisPoints,

solidity/contracts/stBTC.sol

@@ -253,8 +253,11 @@ contract stBTC is ERC4626Fees, PausableOwnable {
         address owner
     ) public override whenNotPaused returns (uint256) {
         uint256 currentAssetsBalance = IERC20(asset()).balanceOf(address(this));
-        if (assets > currentAssetsBalance) {
-            dispatcher.withdraw(assets - currentAssetsBalance);
+        // If there is not enough assets in stBTC to cover user withdrawals and
+        // withdrawal fees then pull the assets from the dispatcher.
+        uint256 assetsWithFees = assets + _feeOnRaw(assets, exitFeeBasisPoints);
+        if (assetsWithFees > currentAssetsBalance) {
+            dispatcher.withdraw(assetsWithFees - currentAssetsBalance);
         }
 
         return super.withdraw(assets, receiver, owner);