WIP: USDC support as first-class citizen

[nicolasbrugneaux]

WIP

This adds support for USDC but it's kinda blocked by a couple things:

• no definite contract for the USDC token (https://docs.celo.org/protocol/transaction/erc20-transaction-fees#alfajores-testnet should behave the same however)
• FeeCurrencyAdapter isn't out in @celo/abis yet

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[arthurgousset]

If I'm not mistaken, this is not going to work for StableToken$USDC, because USDC will not be a core contract, so won't be in Registry.sol.

[nicolasbrugneaux]

righto

[aaronmgdr]

I think my and marianos thoughts here will be helpful for this work

celo-org/celo-monorepo#10910 (comment)

Commenting further on what i said in meeting today. I think we should start thinking in terms of FeeCurrencyWhitelist rather than StableTokens. (separating what were coupled concepts) we are adding support for tokens because of their inclusion in the whitelist. and in theory not every feecurrencywhitelist token needs to be stable.

So rather than adding the token to existing StableToken enums and objects might be time to do more of a thinking on design.

[nicolasbrugneaux]

Closing in favour of #151