[ETHEREUM-CONTRACTS DEVX] Shift fUSDC to 6 decimals

[d10r]

Problem: USDC has 6 decimals while fUSDC has 18.
Applications using fUSDC with the expectation that it behaves like the mainnet counterpart may get burned by this difference.

Solution: change fUSDC to have 6 decimals:

1. change for new deployments: adjust deployment script
2. change for existing deployments: requires a one-time "hack"
   a. add logic to SuperToken which allows the owner to change
   b. update fUSDC to this logic
   c. trigger the logic to make the switch to 6 digits
   d. update to canonical logic

To be considered: effect of that change on existing fUSDCx and upgrading/downgrading.
I currently can't think of any particular side effects. But didn't think about it deeply.

[0xdavinchee]

I would also add permit to USDC, like we did in SuperUpgrader.

[youssefea]

@d10r is this finished?

[hellwolf]

I will bundle this to the EPIC of SuperUpgrader story. When we get there, we need an upgraded super token.

[hellwolf]

let's do it in the code base first, anticipating the upgrade to new testnet when Ethereum is 90 years old.