Staking v2

[rndquu]

We have the old staking contract (which still works) where users are able to stake 3CRV LP tokens and get rewards in UBQ tokens. The old staking contract has some issues:

1. It uses 3CRV LP tokens from the old uAD/3CRV pool. We've redeployed our stable coin so now we're using UUSD instead of the deprecated uAD token.
2. The old staking contract is unaudited
3. There's this issue with the old staking contract

It makes sense to reimplement staking so that users were able to stake UUSD/LUSD LP tokens and get UBQ rewards. This way contributors have an incentive to keep UUSD in the UUSD/LUSD pool instead of swapping UUSD for a more liquid asset.

We've already ported staking to this diamond facet but we've never tested how it really works.

What should be done:

1. Refactor LibStaking to match the original MaterChef implementation
2. Cover LibStaking with unit tests
3. Prepare on-chain migrations

The next step would be to audit the updated code and deploy to production.