util/set: add OfSliceView constructor and Set.AddSliceView

Building a Set from a views.Slice previously required set.Of(v.AsSlice()...),
which allocates an intermediate slice copy before allocating the set. Add
OfSliceView and AddSliceView to populate a set directly from the view,
mirroring the existing AddSlice/AddSeq/AddSet family.

Updates tailscale/corp#45499

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I3f2a9d417c60be8e5f1acd42708e2f9a4d6c1b7e