ux: withdraw return pending fee decrease

[adhusson]

Goal: if the caller knows an old value of a user's credit (or postmaturity credit: credit - pendingFee) he can always do

(...) = updatePosition(lender); 
(...) = withdraw();

and know the new credit (or postmaturity credit) + the part of the (postmaturity) credit change that was due to a loss.

Before this PR, it was only possible by doing 2 additional reads after the last call (or recreating the pending fee formula).

corresponding thread

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[MathisGD]

why shouldn't take return the pendingFee deltas?
should liquidate return the badDebt?

[adhusson]

why shouldn't take return the pendingFee deltas?
should liquidate return the badDebt?

Bad debt unrelated to credit but I can add it.

For deltas i thought i was too misleading due to reentrancy, if you update using just the value before + the delta you can get it wrong.

[adhusson]

this is what it looks like wdyt? #1005

[MathisGD]

discussed: let's do this one and not #1005