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 will be used by the inspector, for showing style sheet contents.
Identifying a specific style sheet is a bit tricky. Depending on where
it came from, a style sheet may have a URL, it might be associated with
a DOM element, both, or neither. This varied information is wrapped in
a new StyleSheetIdentifier struct.