[css-grid] Refactor GridSpan to avoid pointers
Add new boolean to know if a GridSpan is definite or indefinite.
That way we don't need to use pointers and we can always have two
GridSpans in GridCoordinate, if the position is "auto" the GridSpan will
be marked as indefinite. This will allow in a follow-up patch to avoid
repeated calls to methods that resolve positions.
Most operations in GridSpan are restricted to definite GridSpans (access
to positions, iterator, etc.). For indefinite GridSpans we only need to
know that they're indefinite we shouldn't use the rest of the data.
No new tests, no change of behavior.