WIP: fix perf fee principal tracking

[godzillaba]

previously, the following scenario was possible:

1. alice+bob each deposit 100 usdc and get 100 shares
2. subVault doubles in price, so the mastervault has totalAssets == 400, totalPrincipal == 200
3. alice withdraws her 100 shares for 200 usdc, now totalAssets == 200, totalPrincipal == 0
4. fee manager withdraws profit of 200, now there are no assets left and bob is left with no money

    /**
     * @dev Withdraw/redeem common workflow.
     */
    function _withdraw(
        address caller,
        address receiver,
        address _owner,
        uint256 assets,
        uint256 shares
    ) internal virtual override whenNotPaused {
        totalPrincipal -= assets;

        IERC4626 _subVault = subVault;
        if (address(_subVault) != address(0)) {
            _subVault.withdraw(assets, address(this), address(this));
        }

        super._withdraw(caller, receiver, _owner, assets, shares);
    }

this is a WIP attempt to account for fees continuously. it works by changing the internal _convertToAssets and _convertToShares functions to calculate profit and set it aside before proceeding with calculation.

it still needs a function to actually withdraw the fees, but i'm focusing on the accounting for now.