feat: Add minimum first deposit, refactor convert/preview functions (SC-446)

[lucas-manuel]

Share Inflation Attack Initial Approach vs. Final Approach

This PR was originally created to address the issue outlined in picture below within the contract's deposit logic. This was fully implemented in commit d32fd2f. It was subsequently removed in favor of simplifying core logic in the contract and addressing the issue in a different way - by operationally requiring an initial deposit to be made on deployment. This would fix multiple attack vectors as outlined in @hexonaut's comment.

Other Changes within PR

• Adds test to prove that the attack vector is resolved by the initial deposit being made.
• Adds section to README outlining attack prevention measures to be taken.
• Also refactors to use preview functions to return rounding-accurate conversions.
• Updates to have convertToShares and convertToAssets use generic value and asset-specific values.

[linear]

SC-446 Add minimum initial deposit safety measure

[hexonaut]

I've thought about it some more and to keep things simple I don't think we should include any custom logic for ensuring an initial burn amount. There are actually a lot of ways to brick the contract with an initial deposit. This current method prevents the share inflation attack, but you can also just send a non-zero balance to the contract before the first call to deposit to DoS any deposit after that as convertToShares(...) will return 0 always since psm value is > 0 and totalShares = 0.

I think instead let's just make it very clear a small seed amount should be deposited as the first action in all cases. This is what other protocols do, and I think it's easier to not go overly fancy as it introduces complexity.

For the deposit and withdraw functions let's add a receiver param and during deployment we will just deposit some initial amount and send it to the zero address.

As part of the deployment script we can include this initial deposit to force the deployer address to have a seed balance inside it's wallet.

[hexonaut]

Mostly looks good. Some requested changes.

I would also make the recommendation to burn at least 1 full unit instead of 1000 shares.

[hexonaut]

Good to merge now.