Add `Immutable Data` field to the `Spawn Transaction`

[YaronWittenstein]

Depends on: #447

Add to the Spawn Transaction a new field named immutable_data. This field won't hold some ABI data as in verifydata/ctordata/funcdata etc, but it'll hold the Storage Variables belonging to Section #0 (see issue #447).

The issue doesn't involve touching the Global State, merely adding a new field to the Spawn Transaction.