chore: optimise effects that only exist to return a teardown

[Rich-Harris]

There are a number of places where we create effects whose only purpose is to return a teardown function — most notably, every time we create a non-delegated event handler. In these cases we create an unnecessary closure, creating memory overhead, and go through the rigamarole of executing the effect.

It occurred to me that we can optimise this case. Maybe it's an optimisation too far but it seemed like a quick and easy win

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