contracts: Add FeeCurrencyWhitelist

[palango]

Adds the FeeCurrencyWhitelist contract, part of #57.

@pahor167 Please have a look at this. The changes I did:

• Add the license modifier
• Update solidity pragma
• Fix imports
• Removed the public visibility modifier from the constructor, because solc complained.

[palango]

Mhg, I'm not sure why this complains about the ABIs. I'm using the abigen version that is required:

% abigen --version
abigen version 1.10.25-stable

[pahor167]

Mhg, I'm not sure why this complains about the ABIs. I'm using the abigen version that is required:

% abigen --version
abigen version 1.10.25-stable

Did you run make in op-bindings folder after all changes to smart contract were done ? I took the liberty to fetch branch, rebuild bindings and it seems that op-bindings-build is passing.

abigen --version abigen version 1.10.25-stable

My abigen version is same as yours.

[pahor167]

It is necessary to also add FeeCurrencyWhitelist into CeloPredeploys

[palango]

This is included in #70