Support constraints on actions and their parameters in @entry_constraint

[smolkaj]

Since the action of a table is a sum type, this requires some language mechanism for destructing sum types, e.g. ADT-style pattern matching.

We didn't implement this feature thus far since this is a nontrivial effort.

This is related to but distinct from #98, which is about constraining a single action with a new @action_restriction annotation. #98 is simpler to implement since it doesn't require dealing with sum types.