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>

net/dns/manager.go

@@ -19,7 +19,6 @@ import (
 	"sync/atomic"
 	"time"
 
-	xmaps "golang.org/x/exp/maps"
 	"tailscale.com/control/controlknobs"
 	"tailscale.com/health"
 	"tailscale.com/net/dns/resolver"
@@ -31,6 +30,7 @@ import (
 	"tailscale.com/types/logger"
 	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/dnsname"
+	"tailscale.com/util/slicesx"
 )
 
 var (
@@ -204,7 +204,7 @@ func compileHostEntries(cfg Config) (hosts []*HostEntry) {
 	if len(hostsMap) == 0 {
 		return nil
 	}
-	hosts = xmaps.Values(hostsMap)
+	hosts = slicesx.MapValues(hostsMap)
 	slices.SortFunc(hosts, func(a, b *HostEntry) int {
 		if len(a.Hosts) == 0 && len(b.Hosts) == 0 {
 			return 0