op: rename StackOp/Push/Pop to StateOp/Save/Load

The semantics were relaxed in a previous commit; this change renames
to operations accordingly.

API change. Use gofmt to adjust your code accordingly:

gofmt -r 'op.Push(a).Pop() -> op.Save(a).Load()'
gofmt -r 'op.Push(a) -> op.Save(a)'
gofmt -r 'v.Pop() -> v.Load()'
gofmt -r 'op.StackOp -> op.StateOp'

Signed-off-by: Elias Naur <mail@eliasnaur.com>

layout/flex.go

@@ -181,10 +181,10 @@ func (f Flex) Layout(gtx Context, children ...FlexChild) Dimensions {
 				cross = maxBaseline - b
 			}
 		}
-		stack := op.Push(gtx.Ops)
+		stack := op.Save(gtx.Ops)
 		op.Offset(FPt(f.Axis.point(mainSize, cross))).Add(gtx.Ops)
 		child.call.Add(gtx.Ops)
-		stack.Pop()
+		stack.Load()
 		mainSize += f.Axis.Main(dims.Size)
 		if i < len(children)-1 {
 			switch f.Spacing {