util/slicesx: add MapKeys and MapValues from golang.org/x/exp/maps

Importing the ~deprecated golang.org/x/exp/maps as "xmaps" to not
shadow the std "maps" was getting ugly.

And using slices.Collect on an iterator is verbose & allocates more.

So copy (x)maps.Keys+Values into our slicesx package instead.

Updates #cleanup
Updates #12912
Updates #14514 (pulled out of that change)

Change-Id: I5e68d12729934de93cf4a9cd87c367645f86123a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/syspolicy/source/test_store.go

@@ -11,6 +11,7 @@ import (
 	xmaps "golang.org/x/exp/maps"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/set"
+	"tailscale.com/util/slicesx"
 	"tailscale.com/util/syspolicy/internal"
 	"tailscale.com/util/syspolicy/setting"
 )
@@ -418,7 +419,7 @@ func (s *TestStore) NotifyPolicyChanged() {
 		s.mu.RUnlock()
 		return
 	}
-	cbs := xmaps.Values(s.cbs)
+	cbs := slicesx.MapValues(s.cbs)
 	s.mu.RUnlock()
 
 	var wg sync.WaitGroup