This is strictly nicer than passing them around as i32 everywhere,
and by switching to i64 as the underlying type, ID allocation becomes as
simple as incrementing an integer.
This represents each element's set of CSS counters.
https://drafts.csswg.org/css-lists-3/#css-counters-set
Counters are resolved while building the tree. Most elements will not
have any counters to keep track of, so as an optimization, we don't
create a CountersSet object until the element actually needs one.
In order to properly support counters on pseudo-elements, the
CountersSet needs to go somewhere else. However, my experiments with
placing it on the Layout::Node kept hitting a wall. For now, this is
fairly simple at least.
