perf: remove allocations in useReducer

[romgrk]

Summary

I was reading the source for useReducer and I noticed the returned array is always re-allocated. I figured it could be nice to reuse the value if there has been no change.

One possible concern is if the user is doing unholy things with the returned array, e.g.:

function C() {
  const stateResult = useState(0)
  const [state, setState] = stateResult
  const arrayMineNow = stateResult
  arrayMineNow[0] = 42
  arrayMineNow[1] = 42
  return (<button onClick={() => setState(state + 1)}>{state}</button>)
}

But such abnormal usage could be caught by e.g. sealing the array in dev.

How did you test this change?

yarn test and yarn test --prod

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[react-sizebot]

Comparing: a5fc797db14c6e05d4d5c4dbb22a0dd70d41f5d5...96228c8497203c2376b18711327cfe5bff6f2ebe

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	166.62 kB	166.62 kB	=	52.13 kB	52.13 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	176.05 kB	176.05 kB	=	54.97 kB	54.97 kB
facebook-www/ReactDOM-prod.classic.js	=	571.73 kB	571.73 kB	=	100.64 kB	100.64 kB
facebook-www/ReactDOM-prod.modern.js	=	555.46 kB	555.46 kB	=	97.75 kB	97.75 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by :no_entry_sign: dangerJS against 96228c8497203c2376b18711327cfe5bff6f2ebe

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[romgrk]

Ping, if someone is reading PRs. Feel free to decline & close it.

[github-actions[bot]]

This pull request has been automatically marked as stale. If this pull request is still relevant, please leave any comment (for example, "bump"), and we'll keep it open. We are sorry that we haven't been able to prioritize reviewing it yet. Your contribution is very much appreciated.

[github-actions[bot]]

Closing this pull request after a prolonged period of inactivity. If this issue is still present in the latest release, please ask for this pull request to be reopened. Thank you!