Fixes test of parent commit, 6eb97e5.

Also use a type synonym to simplify type signatures in Test.Main

Also add comment to explain why it is this way.

Because Data.Tuple depends on this package, `findWithIndex` can't return
an actual `Tuple`. `ValueWithIndex` exists as a workaround.

Also tweaks the test slightly, and adds a type signature for a local
function.

Divide and conquer traverse 2

Let findWithIndex also return index of found value