Enable tracking the accrued interest

[QGarchery]

Motivated by a remark of @tomrpl, thanks !

Before this PR, the events AccrueFee(uint256 feeShares) and UpdateLastTotalAssets(uint256 newTotalAssets) were emitted at each interaction. Those are not enough to compute the actual yield generated by the vault because:

• feeShares could be 0, if the vault does not take any performance fee we can't recompute the whole interest generated
• newTotalAssets takes into account the funds added by the current interaction. Those funds are not part of the interest generated and are difficult to subtract (it depends on each interaction)

To fix this, the event AccrueFee is transformed into AccrueInterest to add 2 fields:

1. lastTotalAssets, the value of the storage variable of the same name before the interaction. It is provided as a convenience to be able to reconstruct the yield by looking only at events
2. accruedTotalAssets, the total assets in the vault before the interaction, after having accrued the interest

With those, one can compute (accruedTotalAssets - lastTotalAssets) / lastTotalAssets to compute the yield generated during 2 interactions.

[Rubilmax]

Ouf, missed this one thanks

[julien-devatom]

lastTotalAssets can be recovered from the previous event, right?

[Rubilmax]

lastTotalAssets can be recovered from the previous event, right?

No because the totalAssets emitted in this event does not include the eventual deposit/withdrawal that happens after

[QGarchery]

lastTotalAssets can be recovered from the previous event, right?

No because the totalAssets emitted in this event does not include the eventual deposit/withdrawal that happens after

It can be recovered from the last UpdateTotalAssets event though. So it seems like we could remove lastTotalAssets from the AccrueInterest event. How annoying would it be to compute the yield if we did @julien-devatom ?

[julien-devatom]

Its ok while we can recover it from prev event

[QGarchery]

Its ok while we can recover it from prev event

Ok, I'm removing the field lastTotalAssets from AccrueInterest then

[Rubilmax]

Its ok while we can recover it from prev event

Just want to make sure that it's crystal clear: you won't be recovering it from the previous event of the same type: you need to recover it from the previous event of the type UpdateLastTotalAssets @julien-devatom