[core] Add basic support for state invariant

[davidkpiano]

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[Andarist]

we need such tests:

• show the behavior of those invariants for transitions within states that already have invariant, the current tests only show what's the behavior when entering such states
• show that invariant doesn't create a problems when we exit a state that has some invariant requirement (like, the invariant requires context.user but an exit action within the exited state resets the user to null)
• show the interactions with entry, always and parallel

[davidkpiano]

Added most of these