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>

widget/enum.go

@@ -37,9 +37,7 @@ func (e *Enum) Changed() bool {
 
 // Layout adds the event handler for key.
 func (e *Enum) Layout(gtx layout.Context, key string) layout.Dimensions {
-	var st op.StackOp
-	st.Push(gtx.Ops)
-	defer st.Pop()
+	defer op.Push(gtx.Ops).Pop()
 	pointer.Rect(image.Rectangle{Max: gtx.Constraints.Min}).Add(gtx.Ops)
 
 	if index(e.values, key) == -1 {