No GC can occur during `HTMLSlotElement::Assign` and
`Node::assign_slottables_for_a_tree`. Slot assignment involves a couple
DOM traversals and can be very hot, so its worth proving that GC is
impossible and using unrooted iterators.
This shaves around 5% of the runtime of
`tests/blink_perf_tests/perf_tests/shadow_dom/v1-mutate-shallow-tree-then-slot-flatten.html`
(thats on top of servo#44933)
Testing: This should not change existing behaviour and is covered by
tests.
Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>