fix: improve gas cost of hint recovery when inserting into the end of…

… SortedTroves

When trying to create a new Trove that has a lower ICR than the current lowest
ICR (i.e. inserting into the tail of the list), `findInsertPosition()` returns
`(addressOfLowestICRTrove, address(0))`. If we blindly pass these as hints to
`openTrove()` but the transaction ends up pending for so long that the backend
needs to look for a new insert position, we start the traversal in the wrong
direction (from the wrong end of the list), and end up having to traverse over
almost the entire list.

Work around this by replacing the zero address with the other hint.

Workaround for #600.

packages/lib-ethers/src/PopulatableEthersLiquity.ts

@@ -731,7 +731,13 @@ export class PopulatableEthersLiquity
 
     const { hintAddress } = results.reduce((a, b) => (a.diff.lt(b.diff) ? a : b));
 
-    return sortedTroves.findInsertPosition(nominalCollateralRatio.hex, hintAddress, hintAddress);
+    const [prev, next] = await sortedTroves.findInsertPosition(
+      nominalCollateralRatio.hex,
+      hintAddress,
+      hintAddress
+    );
+
+    return prev === AddressZero ? [next, next] : next === AddressZero ? [prev, prev] : [prev, next];
   }
 
   private async _findHints(trove: Trove): Promise<[string, string]> {