Middleware to deep-freeze all state

It'd be cool to have a middleware that automatically makes all items in the store read-only. It'd be similar to the JS package 'deep-freeze', but probably throw errors on mutations instead of silently ignoring them.

Speed of implementation doesn't really matter here, since it'd only be a development tool.

Implementing this in Lua 5.1 might be tricky. Normally for a read-only table, I'd create a new table with __index and __newindex metamethods acting as a proxy, but then you wouldn't be able to iterate (or shallow copy) the table! In 5.2, Lua gained __pairs and __ipairs which would've helped here, but alas.

There might be one edge case in this implementation. Way back when I first used Redux, I would keep JS Promise objects in the store as a way to manage request status. Promise objects have internal mutability, which we might consider to be breaking the Redux paradigm, but because it's not observable outside of attaching a handler to the promise, I don't know if it is. If we wanted to support a use case like that, we'd need to make sure not to deep-freeze some objects, or force Promise objects to be userdata instead.

The issue names no files, tests, or entry points; first locate the store and middleware APIs in the repository and review how state is exposed. Clarify the Lua 5.1 iteration and Promise-like userdata cases before defining completion as development-time mutation errors for nested state.