add: supertoken protocol nft upgrade + celo gas token

[sirpy]

Description

• Add compatibility with latest superfluid NFT upgrade
• Add debitGasFees and creditGasFees for Celo gas token compatability
• code size optimizations: removed unused functions, moved modifiers into inline function calls

See superfluid NFT upgrade:
superfluid-finance/protocol-monorepo#1274

• NOTICE: superfluid added the deployment logic of the new NFTs inside the supertoken logic. Due to contract size issues, we assume that the NFTs will be deployed outside supergooddollar and they could be set using the setNFTProxyContracts method allowed only by the Avatar.
• only relevant contract in the PR is supertoken.sol we also had to change isupertoken.sol to remove unused methods.

See Celo instructions for gas token compatability:

• example implementations https://github.com/celo-org/celo-monorepo/blob/master/packages/protocol/contracts/stability/StableToken.sol
  

[0xdavinchee]

Other than the comments I mentioned, everything else looks in order! new storage is in the correct slots and other logic related to the NFT contracts have been properly added

[0xdavinchee]

The changes you've made here in the SuperGoodDollar and SuperToken contracts as a result of the new Flow NFT feature and v1.5.2 of ethereum-contracts ensures that SuperGoodDollar has access to these new features.

[martinvol]

The name of this parameter is kinda legacy as fees will soon not be sent to the community fund, so maybe we can update that to baseFeeAddress or something like that.

[martinvol]

Took a look at the code and from a high level view, it seems like they do what it's expected from them. We could try to deploy this to an staging testnet to see how it behaves.

[sirpy]

@martinvol how can we deploy that to alfajores?