model, nodestack: bugfix - retain UNINIT actions across executions

Previously, we  were allocating UNINIT actions as snapshotting memory,
just for ease of use. But because they may not immediately trigger a bug
(if they are a valid read-from), such actions must have a longer
lifetime - in fact, a lifetime similar to normal ModelActions.

So, we stash them in NodeStack alongside normal actions.

Also, improve a few ASSERT()'s and clarify a push_front() vs.
push_back() (the list is empty, so it doesn't matter; but it follows the
style of the rest of the lists to use push_front() here).