Regarding pair matching functions for SetDiffer

[spavikevik]

Hi, thanks for the nice library! I have a small issue about the default SetDiffer instance which invokes the following method

difflicious/modules/core/src/main/scala/difflicious/differ/SeqDiffer.scala

Lines 151 to 174 in f4b0c74

	private[difflicious] def diffPairByFunc[A](
	obtained: Seq[A],
	expected: Seq[A],
	func: A => Any,
	itemDiffer: Differ[A],
	): (Vector[DiffResult], Boolean) = {
	val matchedIndexes = mutable.BitSet.empty
	val results = mutable.ArrayBuffer.empty[DiffResult]
	val expWithIdx = expected.zipWithIndex
	var allIsOk = true
	obtained.foreach { a =>
	val aMatchVal = func(a)
	val found = expWithIdx.find {
	case (e, idx) =>
	if (!matchedIndexes.contains(idx) && aMatchVal == func(e)) {
	val res = itemDiffer.diff(a, e)
	results += res
	matchedIndexes += idx
	allIsOk &= res.isOk
	true
	} else {
	false
	}
	}

The func passed in the default instance is identity, which results in pairs of matching elements paired by equality.

However, this amounts to a useless Differ instance unless one specifies a custom pairing function. I understand the utility of being able to specify a custom pairing function for an unordered collection but I'd also expect a reasonable default one that is smart enough to match elements that are similar, something akin to the example here:
softwaremill/diffx#6 (comment)

Or alternatively, it would be better to drop the default instance.