This fixes an issue where refinements were not possible due to the shallow equality check there.

Before, steps did not accurately model the action that was executed, in a formal sense. Instead, they stored the action that was in the prefix term, and for input actions, a special step was used to store the value.
Now, CCSAction can either represent an action in a prefix term, or an executed action. Flags indicate which action is valid where, and CCSInput and CCSMatch actions turn into CCSExecutedInput when executing them.