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

ipn/backend.go

@@ -11,6 +11,7 @@ import (
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/empty"
+	"tailscale.com/types/structs"
 	"tailscale.com/wgengine"
 )
 
@@ -46,6 +47,7 @@ type NetworkMap = controlclient.NetworkMap
 // that they have not changed.
 // They are JSON-encoded on the wire, despite the lack of struct tags.
 type Notify struct {
+	_             structs.Incomparable
 	Version       string           // version number of IPN backend
 	ErrMessage    *string          // critical error message, if any
 	LoginFinished *empty.Message   // event: non-nil when login process succeeded

ipn/message.go

@@ -14,6 +14,7 @@ import (
 	"time"
 
 	"tailscale.com/types/logger"
+	"tailscale.com/types/structs"
 	"tailscale.com/version"
 )
 
@@ -34,6 +35,7 @@ type FakeExpireAfterArgs struct {
 // Command is a command message that is JSON encoded and sent by a
 // frontend to a backend.
 type Command struct {
+	_       structs.Incomparable
 	Version string
 
 	// Exactly one of the following must be non-nil.