util/IntrusiveHashSet: replace template parameters Hash/Equal with a single one

Preparing to add a key extraction function.  Without this "Operators"
template parameter, we'd have even more template parameters, and that
parameter list would grow too complex.  Better wrap it in one single
template that contains all operators.

This is an API change which all callers need to adjust to, but it will
be worth it.

test/util/TestIntrusiveHashSet.cxx

@@ -38,7 +38,9 @@ TEST(IntrusiveHashSet, Basic)
 {
 	IntItem a{1}, b{2}, c{3}, d{4}, e{5}, f{1};
 
-	IntrusiveHashSet<IntItem, 3> set;
+	IntrusiveHashSet<IntItem, 3,
+			 IntrusiveHashSetOperators<IntItem::Hash,
+						   IntItem::Equal>> set;
 
 	{
 		auto [position, inserted] = set.insert_check(2);