Changing access of _feeOnRaw and _feeOnTotal to internal

We need to calculate fees that are associated with withdrawal assets
upon withdrawing when calculating how much assets should be withdarawn
from the dispatcher contract. Instead of calling a preview and
conversion functions we can modify access modifiers of _feeOnRaw and
_feeOnTotal to internal and use it in stBTC. That would simplify
calculations.

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

@@ -255,7 +255,8 @@ contract stBTC is ERC4626Fees, PausableOwnable {
         uint256 currentAssetsBalance = IERC20(asset()).balanceOf(address(this));
         // If there is not enough assets in stBTC to cover user withdrawals and
         // withdrawal fees then pull the assets from the dispatcher.
-        uint256 assetsWithFees = convertToAssets(previewWithdraw(assets));
+        uint256 assetsWithFees = assets +
+            _feeOnRaw(assets, _exitFeeBasisPoints());
         if (assetsWithFees > currentAssetsBalance) {
             dispatcher.withdraw(assetsWithFees - currentAssetsBalance);
         }