chore: make effect scheduling more efficient

[Rich-Harris]

probably won't merge this due to conflicts with async, which is where I noticed this, but I thought it worth pushing up so that I don't lose track of it

We currently have some inefficiency around effect scheduling. Inside certain blocks, like each and await blocks, we update internal sources that populate the branches within. Updating these sources causes a schedule_effect to occur while the flush is ongoing, but since the parent effects have already been marked clean by the time the child effect is scheduled, this causes the root effect to be re-added to queued_root_effects, which in turn causes another turn of the while loop in flush_queued_root_effects. Since the newly scheduled effects are already handled as part of the first flush, the second flush is just a wasteful no-op.

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