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.
d70caf1da1