Keep track of what RTIs a Result node is scanning.

Keep track of what RTIs a Result node is scanning.

Result nodes now include an RTI set, which is only non-NULL when they
have no subplan, and is taken from the relid set of the RelOptInfo
that the Result is generating.

Using that information, EXPLAIN now emits, where relevant, a "Replaces" line
that says whether it replaced a scan, a join, or an aggregate; and in the
former two cases, which relations were involved.

Likewise, pg_overexplain's EXPLAIN (RANGE_TABLE) now displays the RTIs
stored in a Result node just as it already does for other RTI-bearing
node types.