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>

wgengine/magicsock/endpoint.go

@@ -21,7 +21,6 @@ import (
 	"sync/atomic"
 	"time"
 
-	xmaps "golang.org/x/exp/maps"
 	"golang.org/x/net/ipv4"
 	"golang.org/x/net/ipv6"
 	"tailscale.com/disco"
@@ -34,6 +33,7 @@ import (
 	"tailscale.com/types/logger"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/ringbuffer"
+	"tailscale.com/util/slicesx"
 )
 
 var mtuProbePingSizesV4 []int
@@ -587,7 +587,7 @@ func (de *endpoint) addrForWireGuardSendLocked(now mono.Time) (udpAddr netip.Add
 	needPing := len(de.endpointState) > 1 && now.Sub(oldestPing) > wireguardPingInterval
 
 	if !udpAddr.IsValid() {
-		candidates := xmaps.Keys(de.endpointState)
+		candidates := slicesx.MapKeys(de.endpointState)
 
 		// Randomly select an address to use until we retrieve latency information
 		// and give it a short trustBestAddrUntil time so we avoid flapping between