Revisit caching of attribute values

[Rich-Harris]

Describe the bug
#3723 causes a performance regression — the getAttribute is too costly, it turns out. Man, the DOM sucks.

Severity
Needs to get fixed before the next stable

Additional context
Since #3579 was directed at solving a specific case, maybe we could just address that case directly — i.e. treat hydration differently

[mrkishi]

Since template values are already checked for changes on runtime, I don't think attr really needs that check and the simplest solution would be to just move it to h().

I wonder if we could pull off skipping h altogether?

const target = document.body;
const props = { a: 1, b: 2 };

const { head, html, css, uuid } = ServerComponent.render(props);
// first element: <tag data-svelte-uuid=1231>

target.innerHTML = html;

// we use `uuid` when we know props is the same
const component = new ClientComponent({ target, hydrate: true, props, uuid });

/*
    l(nodes) {
        tag = claim_element(nodes, "tag", { uuid: true }, false);
        ...
        if (tag.dataset.svelteUuid !== uuid) this.h();
    }
*/

[benmccann]

It looks like this was addressed awhile back