core: move GetScriptContainer to runtime

It also brings ToStackItem to Block and Transaction, previously this was
avoided to separate block and transaction packages from VM. But turns out
`transaction` depends on `stackitem` already, so this makes little sense (but
can be shuffled in another way if needed).

Context.Container is still a hash.Hashable because we have a number of
occasions (header or MPT root verification) where there is no ToStackItem
implementation possible. Maybe they can go with `nil` Container, but I don't
want to have this risk for now.

pkg/core/interop/runtime/engine.go

@@ -10,6 +10,10 @@ import (
 	"go.uber.org/zap"
 )
 
+type itemable interface {
+	ToStackItem() stackitem.Item
+}
+
 const (
 	// MaxEventNameLen is the maximum length of a name for event.
 	MaxEventNameLen = 32
@@ -37,6 +41,17 @@ func GetEntryScriptHash(ic *interop.Context) error {
 	return ic.VM.PushContextScriptHash(ic.VM.Istack().Len() - 1)
 }
 
+// GetScriptContainer returns transaction or block that contains the script
+// being run.
+func GetScriptContainer(ic *interop.Context) error {
+	c, ok := ic.Container.(itemable)
+	if !ok {
+		return errors.New("unknown script container")
+	}
+	ic.VM.Estack().PushItem(c.ToStackItem())
+	return nil
+}
+
 // Platform returns the name of the platform.
 func Platform(ic *interop.Context) error {
 	ic.VM.Estack().PushItem(stackitem.NewByteArray([]byte("NEO")))