Improve relation width estimation for subqueries.
As per the ancient comment for set_rel_width, it really wasn't much good for relations that aren't plain tables: it would never find any stats and would always fall back on datatype-based estimates, which are often pretty silly. Fix that by copying up width estimates from the subquery planning process. At some point we might want to do this for CTEs too, but that would be a significantly more invasive patch because the sub-PlannerInfo is no longer accessible by the time it's needed. I refrained from doing anything about that, partly for fear of breaking the unmerged CTE-related patches. In passing, also generate less bogus width estimates for whole-row Vars. Per a gripe from Jon Nelson.
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