types/structs: add structs.Incomparable annotation, use it where applicable

Shotizam before and output queries:

sqlite> select sum(size) from bin where func like 'type..%';
129067
=>
120216

control/controlclient/auto.go

@@ -22,6 +22,7 @@ import (
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/empty"
 	"tailscale.com/types/logger"
+	"tailscale.com/types/structs"
 )
 
 // TODO(apenwarr): eliminate the 'state' variable, as it's now obsolete.
@@ -61,6 +62,7 @@ func (s state) String() string {
 }
 
 type Status struct {
+	_             structs.Incomparable
 	LoginFinished *empty.Message
 	Err           string
 	URL           string
@@ -94,6 +96,7 @@ func (s Status) String() string {
 }
 
 type LoginGoal struct {
+	_            structs.Incomparable
 	wantLoggedIn bool          // true if we *want* to be logged in
 	token        *oauth2.Token // oauth token to use when logging in
 	flags        LoginFlags    // flags to use when logging in

control/controlclient/controlclient_test.go

@@ -13,7 +13,9 @@ import (
 
 func fieldsOf(t reflect.Type) (fields []string) {
 	for i := 0; i < t.NumField(); i++ {
-		fields = append(fields, t.Field(i).Name)
+		if name := t.Field(i).Name; name != "_" {
+			fields = append(fields, name)
+		}
 	}
 	return
 }

control/controlclient/direct.go

@@ -30,10 +30,12 @@ import (
 	"tailscale.com/net/tlsdial"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/logger"
+	"tailscale.com/types/structs"
 	"tailscale.com/version"
 )
 
 type Persist struct {
+	_                 structs.Incomparable
 	PrivateMachineKey wgcfg.PrivateKey
 	PrivateNodeKey    wgcfg.PrivateKey
 	OldPrivateNodeKey wgcfg.PrivateKey // needed to request key rotation