math logic + internal fn

https://github.com/hashedMae/mentorshipProjects/pull/4

Instead of repeating math logic, you should call the convertToAssets function.

Even better, for gas cost. You should put the logic on an internal _convertToAssets function, and call _convertToAssets from the external convertToAssets (which becomes a user-facing function), mint and redeem.

The same pattern should be followed for convertToShares, deposit and withdraw.