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/lru/lru_test.go

@@ -10,7 +10,7 @@ import (
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
-	xmaps "golang.org/x/exp/maps"
+	"tailscale.com/util/slicesx"
 )
 
 func TestLRU(t *testing.T) {
@@ -75,7 +75,7 @@ func TestStressEvictions(t *testing.T) {
 	for len(vm) < numKeys {
 		vm[rand.Uint64()] = true
 	}
-	vals := xmaps.Keys(vm)
+	vals := slicesx.MapKeys(vm)
 
 	c := Cache[uint64, bool]{
 		MaxEntries: cacheSize,
@@ -106,7 +106,7 @@ func TestStressBatchedEvictions(t *testing.T) {
 	for len(vm) < numKeys {
 		vm[rand.Uint64()] = true
 	}
-	vals := xmaps.Keys(vm)
+	vals := slicesx.MapKeys(vm)
 
 	c := Cache[uint64, bool]{}