op: add op.Push and op.Record funcs

The funcs replace stack.Push and macro.Record, which become private.
This makes stack and macro faster to write, in particular for stacks
where you can just write the following line to save and restore the
state :

  defer op.Push(ops).Pop()

This usage requires Push to return a pointer (since Pop has a pointer
receiver), or else the code doesn't compile.

For consistancy, I tried to do the same for op.Record, but this implied
to turn all the MacroOp fields into pointers, and this caused some
panics. As a result, op.Record doesn't return a pointer.

An other side effect pointed by Larry Clapp: StackOp and MacroOp are not
re-usable any more, you have to allocate a new one for each usage, using
the described funcs above.

Signed-off-by: Thomas Bruyelle <thomas.bruyelle@gmail.com>

io/router/pointer_test.go

@@ -311,7 +311,6 @@ func TestPointerEnterLeaveNested(t *testing.T) {
 
 func TestPointerActiveInputDisappears(t *testing.T) {
 	handler1 := new(int)
-	// Save this logic so we can redo it later.
 	var ops op.Ops
 	var r Router
 
@@ -386,9 +385,7 @@ func TestMultitouch(t *testing.T) {
 // addPointerHandler adds a pointer.InputOp for the tag in a
 // rectangular area.
 func addPointerHandler(ops *op.Ops, tag event.Tag, area image.Rectangle) {
-	var stack op.StackOp
-	stack.Push(ops)
-	defer stack.Pop()
+	defer op.Push(ops).Pop()
 	pointer.Rect(area).Add(ops)
 	pointer.InputOp{Tag: tag}.Add(ops)
 }