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>

widget/material/button.go

@@ -271,7 +271,7 @@ func drawInk(gtx layout.Context, c widget.Press) {
 	alpha := 0.7 * alphaBezier
 	const col = 0.8
 	ba, bc := byte(alpha*0xff), byte(col*0xff)
-	defer op.Push(gtx.Ops).Pop()
+	defer op.Save(gtx.Ops).Load()
 	rgba := f32color.MulAlpha(color.NRGBA{A: 0xff, R: bc, G: bc, B: bc}, ba)
 	ink := paint.ColorOp{Color: rgba}
 	ink.Add(gtx.Ops)