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>

cmd/tailscale/cli/serve_v2.go

@@ -28,6 +28,7 @@ import (
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/tailcfg"
 	"tailscale.com/util/mak"
+	"tailscale.com/util/slicesx"
 	"tailscale.com/version"
 )
 
@@ -439,11 +440,7 @@ func (e *serveEnv) messageForPort(sc *ipn.ServeConfig, st *ipnstate.Status, dnsN
 	}
 
 	if sc.Web[hp] != nil {
-		var mounts []string
-
-		for k := range sc.Web[hp].Handlers {
-			mounts = append(mounts, k)
-		}
+		mounts := slicesx.MapKeys(sc.Web[hp].Handlers)
 		sort.Slice(mounts, func(i, j int) bool {
 			return len(mounts[i]) < len(mounts[j])
 		})