types/key: add MachinePrivate and MachinePublic.

Plumb throughout the codebase as a replacement for the mixed use of
tailcfg.MachineKey and wgkey.Private/Public.

Signed-off-by: David Anderson <danderson@tailscale.com>

ipn/ipnlocal/state_test.go

@@ -17,6 +17,7 @@ import (
 	"tailscale.com/syncs"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/empty"
+	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/netmap"
 	"tailscale.com/types/persist"
@@ -93,7 +94,7 @@ type mockControl struct {
 	calls       []string
 	authBlocked bool
 	persist     persist.Persist
-	machineKey  wgkey.Private
+	machineKey  key.MachinePrivate
 }
 
 func newMockControl() *mockControl {