Avoid re-encoding old state

[martosaur]

Currently, LiveState encodes both old and new state before diffing. This becomes a bottleneck if the state is updated frequently, e.g. if we're streaming a lot of small updates (LLM response deltas 😄 )

One optimization that can be made is to store encoded old state in the socket assigns so we wouldn't need to re-encode it again. That was initially my plan, but I quickly ran into a wall. MessageBuilder is a slightly awkward abstraction. Looking at the docs, it's main purpose is to make diffing algorithm swappable, but it does more than that:

• it handles encoding, which is technically swappable, but it's unclear if that's intentional. This protocol is core to the library.
• it defines message shape, which isn't swappable as JS library code relies on it.

So I went ahead and merged both of these things into the channel itself and made JSONDiff.diff/2 function replaceable with diff_function channel option. I also made channel not increment version and send empty patch if an updated resulted in an empty diff. There was also a bug in push_state_change which used overridable backend state keys for message that was supposed to be sent to the frontend.

Obviously, since message_builder option is deleted, this is a backwards incompatible change. So it's completely at your mercy if you want to go with major version.

[the code in this PR was slightly LLM assisted]