Show values of SET statements as constants in pg_stat_statements

This is a continuation of work like 11c34b342b, done to reduce the
bloat of pg_stat_statements by applying more normalization to query
entries.  This commit is able to detect and normalize values in
VariableSetStmt, resulting in:
SET conf_param = $1

Compared to other parse nodes, VariableSetStmt is embedded in much more
places in the parser, impacting many query patterns in
pg_stat_statements.  A custom jumble function is used, with an extra
field in the node to decide if arguments should be included in the
jumbling or not, a location field being not enough for this purpose.
This approach allows for a finer tuning.

Clauses relying on one or more keywords are not normalized, for example:
* DEFAULT
* FROM CURRENT
* List of keywords.  SET SESSION CHARACTERISTICS AS TRANSACTION,
where it is critical to differentiate different sets of options, is a
good example of why normalization should not happen.

Some queries use VariableSetStmt for some subclauses with SET, that also
have their values normalized:
- ALTER DATABASE
- ALTER ROLE
- ALTER SYSTEM
- CREATE/ALTER FUNCTION

ba90eac7a9 has added test coverage for most of the existing SET
patterns.  The expected output of these tests shows the difference this
commit creates.  Normalization could be perhaps applied to more portions
of the grammar but what is done here is conservative, and good enough as
a starting point.

Author: Greg Sabino Mullane, Michael Paquier
Discussion: https://postgr.es/m/36e5bffe-e989-194f-85c8-06e7bc88e6f7@amazon.com
Discussion: https://postgr.es/m/B44FA29D-EBD0-4DD9-ABC2-16F1CB087074@amazon.com
Discussion: https://postgr.es/m/CAKAnmmJtJY2jzQN91=2QAD2eAJAA-Per61eyO48-TyxEg-q0Rg@mail.gmail.com

contrib/pg_stat_statements/expected/wal.out

@@ -18,7 +18,7 @@ FROM pg_stat_statements ORDER BY query COLLATE "C";
  DELETE FROM pgss_wal_tab WHERE a > $1                        |     1 |    1 | t                   | t                     | t
  INSERT INTO pgss_wal_tab VALUES(generate_series($1, $2), $3) |     1 |   10 | t                   | t                     | t
  SELECT pg_stat_statements_reset() IS NOT NULL AS t           |     1 |    1 | f                   | f                     | f
- SET pg_stat_statements.track_utility = FALSE                 |     1 |    0 | f                   | f                     | t
+ SET pg_stat_statements.track_utility = $1                    |     1 |    0 | f                   | f                     | t
  UPDATE pgss_wal_tab SET b = $1 WHERE a > $2                  |     1 |    3 | t                   | t                     | t
 (5 rows)