fix: work around huge gas cost when reducing ICR of bottom Trove

[danielattilasimon]

The problem:

[...] for the last trove in the list, the addressOfLowestICRTrove is their own address - and the reinsertion first removes it before the insert. The system can't find it in the list, so starts at the opposite end, and runs out of gas.

DefiSaver's original workaround:

[...] check for this specific edge case: if the trove is last in the list, use the 2nd last trove's address as both hints.

This implements a generalized version of that workaround:

• In either of the hints is the user's own Trove, "jump over" it and replace it with the next/prev Trove.
• If either of the hints is address(0), replace it with the other hint.

Fixes #659.

[edmulraney]

⚡

[github-actions]

🎉 This PR is included in version @liquity/lib-ethers-v3.3.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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