chore: simplify updates

[Rich-Harris]

There's more where this came from, but this is the immediate low-hanging fruit.

At the moment we separate things into an 'init' and 'update' phase but it's really quite unnecessary — we can make our lives simpler by putting everything except groupable render effects into the init phase.

I have a hunch that this will eventually also unlock a simpler hydration algorithm (that doesn't for example rely on $$fragment) but this is worthwhile independently of that.

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