util/set: add Of variant of SetOf that takes variadic parameter

set.Of(1, 2, 3) is prettier than set.SetOf([]int{1, 2, 3}).

I was going to change the signature of SetOf but then I noticed its
name has stutter anyway, so I kept it for compatibility. People can
prefer to use set.Of for new code or slowly migrate.

Also add a lazy Make method, which I often find myself wanting,
without having to resort to uglier mak.Set(&set, k, struct{}{}).

Updates #cleanup

Change-Id: Ic6f3870115334efcbd65e79c437de2ad3edb7625
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/set/set.go

@@ -14,6 +14,11 @@ type Set[T comparable] map[T]struct{}
 
 // SetOf returns a new set constructed from the elements in slice.
 func SetOf[T comparable](slice []T) Set[T] {
+	return Of(slice...)
+}
+
+// Of returns a new set constructed from the elements in slice.
+func Of[T comparable](slice ...T) Set[T] {
 	s := make(Set[T])
 	s.AddSlice(slice)
 	return s
@@ -41,6 +46,13 @@ func (s Set[T]) AddSet(es Set[T]) {
 	}
 }
 
+// Make lazily initializes the map pointed to by s to be non-nil.
+func (s *Set[T]) Make() {
+	if *s == nil {
+		*s = make(Set[T])
+	}
+}
+
 // Slice returns the elements of the set as a slice. The elements will not be
 // in any particular order.
 func (s Set[T]) Slice() []T {