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/stack.go

@@ -104,10 +104,10 @@ func (s Stack) Layout(gtx Context, children ...StackChild) Dimensions {
 		case SW, S, SE:
 			p.Y = maxSZ.Y - sz.Y
 		}
-		stack := op.Push(gtx.Ops)
+		stack := op.Save(gtx.Ops)
 		op.Offset(FPt(p)).Add(gtx.Ops)
 		ch.call.Add(gtx.Ops)
-		stack.Pop()
+		stack.Load()
 		if baseline == 0 {
 			if b := ch.dims.Baseline; b != 0 {
 				baseline = b + maxSZ.Y - sz.Y - p.Y