Add rari-fuse strategy [rari-fuse]

[MantisClone]

Changes proposed in this pull request:

• Add a rari-fuse strategy that returns the voter's underlying collateral balances in a given Rari Fuse market (fToken).

[MantisClone]

Unfortunately, both of the FusePoolLens functions that would return a user's underlying collateral balance revert, and seem to be broken: getPoolUsersWithData and getPoolAssetsWithData

So I'm attempting to reverse engineer them inside the Snapshot Strategy using lower-level calls to the pool (Comptroller) and the cToken.

Reference: https://github.com/Rari-Capital/fuse-v1/blob/b5c269208f48ab413d816ccd5f67bf79caea73fd/src/FusePoolLens.sol#L252-L349

[MantisClone]

Reading through the FusePoolLens.sol code, I think I can get a user's supplied balance directly from the cToken using:

https://github.com/Rari-Capital/fuse-v1/blob/b5c269208f48ab413d816ccd5f67bf79caea73fd/src/FusePoolLens.sol#L319

            asset.supplyBalance = cToken.balanceOfUnderlying(user);

[MantisClone]

After further study of the cToken implementation on etherscan: https://etherscan.io/address/0x67db14e73c2dce786b5bbbfa4d010deab4bbfcf9#code, I realized that cToken.balanceOfUnderlying(user) doesn't exist on-chain. Instead, cToken.balanceOf(user) can be used.

This means that I could plug the cTokenAddress into the standard erc20-balance-of strategy and it would work the same way! 🤦

Nonetheless, I decided to finish off the PR by changing the variable name from address to cTokenAddress and to update the README to make it rari-fuse specific.

[MantisClone]

After spending 8 hours working on this, I realized that getting a user's balance in a given Rari Fuse market is as simple as calling cToken.balanceOf(user), and that the same results could be achieved using existing strategies:

1. erc20-balance-of strategy by plugging the cToken address into the address parameter.
2. cToken strategy which adds an optional waiting period between minting and voting, and the option to exclude borrowers by granting them 0 voting weight.

I couldn't find precedents for duplicate strategies.

I briefly considered creating a PR that contains only the rari-fuse README.md which then points the users to the erc20-balance-of and ctoken strategies. But I don't know if this would be compatible with the snapshot app.

I also wondered if rari-fuse could be an alias of erc20-balance-of but then I saw #107 which dismisses the idea of aliases.

Considering all this - I decided to simply close this PR and move on.

[MantisClone]

I realized my last comment was wrong. In Rari Fuse, there's an exchange rate between the fToken (what I've previously been calling cToken) and the underlying token. So I'm thinking this strategy may still be needed.

https://docs.rari.capital/fuse/#interpreting-exchange-rates