chore: use closures for state proxies

[Rich-Harris]

alternative to #12868. This doesn't yet change any behaviour (apart from the one slightly odd test that has been updated), this is just to get a sense of the performance implications

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[trueadm]

Looks impressive. Here's the benchmark I was using.

Toggle create 10k rows and clear.

[Rich-Harris]

How does that compare to main?

[Rich-Harris]

(also, what of the memory impact — manageable?)

[trueadm]

This is main. Memory usage is up by about 3-5%, nothing major.

[Rich-Harris]

Since the performance characteristics look okay, I've expanded on this in #12916