Strategy lever PoC

[iamsahu]

No description provided.

[alcueca]

Instead of a review, this time I chose to do some coding. It's a good sign, because it means that I'm finally comfortable with the code.

In this branch, I fixed most of the imports and a warning, but more importantly I fixed approvals.

We always do an ERC3156 flash loan, meaning that we always have to approve for the flash loan repayment. We get the token address in the onFlashLoan invocation, and we trust the lender, so we can just approve all flash loan repayments like this:

In PR11 I made two deeper changes:

1. Every time that we repay debt in the Yield Protocol, we can send the base assets to the appropriate join, or the fyToken to the appropriate fyToken contract, and then call ladle.pour, ladle.repay, or ladle.close. Removes approvals and makes the code more consistent.
2. You can't call burnForBase after maturity, so I coded a new redeem option that redeems the fyToken. I kept the close option because it should be useful if pool rates are low, and users can't use repay to unwind.

[alcueca]

I didn't see exhaustive tests, so I would suggest that we start with those. There were bugs in there that would have wasted time from the frontend team.

[alcueca]

We can test a bit further.

[alcueca]

Well done. Let's merge and see it live.