Show names of DEALLOCATE as constants in pg_stat_statements

This commit switches query jumbling so as prepared statement names are
treated as constants in DeallocateStmt.  A boolean field is added to
DeallocateStmt to make a distinction between ALL and named prepared
statements, as "name" was used to make this difference before, NULL
meaning DEALLOCATE ALL.

Prior to this commit, DEALLOCATE was not tracked in pg_stat_statements,
for the reason that it was not possible to treat its name parameter as a
constant.  Now that query jumbling applies to all the utility nodes,
this reason does not apply anymore.

Like 638d42a3c5, this can be a huge advantage for monitoring where
prepared statement names are randomly generated, preventing bloat in
pg_stat_statements.  A couple of tests are added to track the new
behavior.

Author: Dagfinn Ilmari Mannsåker, Michael Paquier
Reviewed-by: Julien Rouhaud
Discussion: https://postgr.es/m/ZMhT9kNtJJsHw6jK@paquier.xyz

contrib/pg_stat_statements/expected/utility.out

@@ -472,6 +472,47 @@ SELECT pg_stat_statements_reset();
  
 (1 row)
 
+-- Execution statements
+SELECT 1 as a;
+ a 
+---
+ 1
+(1 row)
+
+PREPARE stat_select AS SELECT $1 AS a;
+EXECUTE stat_select (1);
+ a 
+---
+ 1
+(1 row)
+
+DEALLOCATE stat_select;
+PREPARE stat_select AS SELECT $1 AS a;
+EXECUTE stat_select (2);
+ a 
+---
+ 2
+(1 row)
+
+DEALLOCATE PREPARE stat_select;
+DEALLOCATE ALL;
+DEALLOCATE PREPARE ALL;
+SELECT calls, rows, query FROM pg_stat_statements ORDER BY query COLLATE "C";
+ calls | rows |                 query                 
+-------+------+---------------------------------------
+     2 |    0 | DEALLOCATE $1
+     2 |    0 | DEALLOCATE ALL
+     2 |    2 | PREPARE stat_select AS SELECT $1 AS a
+     1 |    1 | SELECT $1 as a
+     1 |    1 | SELECT pg_stat_statements_reset()
+(5 rows)
+
+SELECT pg_stat_statements_reset();
+ pg_stat_statements_reset 
+--------------------------
+ 
+(1 row)
+
 -- SET statements.
 -- These use two different strings, still they count as one entry.
 SET work_mem = '1MB';