vm: embed reference counter in compound items

```
name              old time/op  new time/op  delta
RefCounter_Add-8  44.8ns ± 4%  11.7ns ± 3%  -73.94%  (p=0.000 n=10+10)
```

Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>

pkg/vm/stackitem/reference.go

@@ -0,0 +1,15 @@
+package stackitem
+
+type rc struct {
+	count int
+}
+
+func (r *rc) IncRC() int {
+	r.count++
+	return r.count
+}
+
+func (r *rc) DecRC() int {
+	r.count--
+	return r.count
+}