Address review: keys by reference, comments with examples, coverage in slt

- extract_partition_keys returns HashSet<&Expr>; no clone of the key exprs
- comments no longer describe the previous behavior; the helper's doc gives
  examples of what is and is not pushed under PARTITION BY a, b + c
- the DataFrame-built unit tests are replaced by EXPLAIN + result cases in
  push_down_filter_regression.slt (expression key, key used twice, mixed
  column and expression keys, operand order, several windows with common
  and with disjoint keys, subqueries, volatile predicates); one unit test
  remains for the expression key

Author: zhuqi-lucas