Cache JestHook emitters

[Connormiha]

Summary

Just cached JestHookSubscriber and JestHookEmitter to avoid creating new object on each getSubscriber(), getEmitter() calls.

Test plan

[scotthovestadt]

The idea makes sense. Want to take it a step further and switch the data structure of this._listeners to a Map for more perf wins?

[Connormiha]

The idea makes sense. Want to take it a step further and switch the data structure of this._listeners to a Map for more perf wins?

This may impair readability and convenience.

hooks.getEmitter().onTestRunComplete(results);

vs

hooks.getEmitter().get('onTestRunComplete')(results);

[rogeliog]

LGTM

[scotthovestadt]

Let's make sure this gets in for the release-- just needs a changelog.

[Connormiha]

Let's make sure this gets in for the release-- just needs a changelog.

done @scotthovestadt

[github-actions]

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