chore: optimise attributes

[Rich-Harris]

In this PR, attribute values are stored on an element.__attributes object, which allows us to determine whether or not an attribute needs to be updated without touching the DOM.

It will enable us to get rid of the if (div_blah !== (div_blah = whatever())) stuff, which — as well as making compiler output more efficient — will make render effects more performant by avoiding the deopts that result from closures referencing values that get reassigned. (At least, that's what @trueadm tells me.) That will happen in a follow-up PR.

By calling element.getAttribute(...) exactly once, during hydration, we also solve #10914. I guess I should add a test for that. actually no, this doesn't fix that issue yet — need to get rid of the if blocks first

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[dummdidumm]

This is missing the DEV condition

[Rich-Harris]

These attributes are skipped in dev and prod mode. The only difference is that in dev mode we additionally warn, which is what happens on the next line