Prevent using mutable parts of the action object

[adambiltcliffe]

In development mode, we should find a way to protect the user from passing in an object that contains e.g. an array and then returning that array as part of the state, because it will not be frozen like the rest of the state. (Example: if it's modified during the playAction call, then board-state will use the modified version when testing replayability, which produces very misleading errors.)