types/mapx, ipn/ipnext: add ordered map, akin to set.Slice

We had an ordered set type (set.Slice) already but we occasionally want
to do the same thing with a map, preserving the order things were added,
so add that too, as mapsx.OrderedMap[K, V], and then use in ipnext.

Updates #12614

Change-Id: I85e6f5e11035571a28316441075e952aef9a0863
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/extension_host.go

@@ -162,17 +162,14 @@ func newExtensionHost(logf logger.Logf, b Backend, overrideExts ...*ipnext.Defin
 	}
 
 	// Use registered extensions.
-	exts := ipnext.Extensions().All()
-	numExts := ipnext.Extensions().Len()
+	extDef := ipnext.Extensions()
 	if overrideExts != nil {
 		// Use the provided, potentially empty, overrideExts
 		// instead of the registered ones.
-		exts = slices.All(overrideExts)
-		numExts = len(overrideExts)
+		extDef = slices.Values(overrideExts)
 	}
 
-	host.allExtensions = make([]ipnext.Extension, 0, numExts)
-	for _, d := range exts {
+	for d := range extDef {
 		ext, err := d.MakeExtension(logf, b)
 		if errors.Is(err, ipnext.SkipExtension) {
 			// The extension wants to be skipped.