Optimize size of lex structures

LEX, st_select_lex, st_select_unit optimized for space:
- Use bit fields for bool variables
- Ensure that all bit fields are initialized (improves
  performance for init functions as all bit fields can be
  initalized with one memory access)
- Move members around in above structures to remove alignment
  gaps

Some savings:
LEX: 7032 -> 6880
THD: 25608 -> 25456
st_select_lex_unit: 2048 -> 2008

LEX::start():                    1321 -> 1245 instructions
st_select_lex_unit::init_query()  284 ->  214 instructions
st_select_lex::init_query():      766 ->  692 instructions
st_select_lex::init_select():     563 ->  540 instructions

Other things:
- Removed not used LEX::select_allow_into
- Fixed MDEV-25510 Assertion `sel->select_lock ==
   st_select_lex::select_lock_type::NONE' which was caused by this commit.

mysql-test/main/parser.result

@@ -2105,3 +2105,28 @@ ERROR HY000: Unknown data type: 'ACTION'
 #
 # End of 10.5 tests
 #
+#
+# Start of 10.6 tests
+#
+#
+# MDEV-25510 Assertion `sel->select_lock ==
+# st_select_lex::select_lock_type::NONE' failed in Lex_select_lock::set_to
+#
+(SELECT x FROM t WINDOW w1 AS () FOR UPDATE) LIMIT 1;
+ERROR 42S02: Table 'test.t' doesn't exist
+create table t1 (x int);
+insert into t1 values (1),(2);
+explain extended (SELECT x FROM t1 WINDOW w1 as () FOR UPDATE) LIMIT 1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	100.00	
+Warnings:
+Note	1003	(select `test`.`t1`.`x` AS `x` from `test`.`t1` limit 1)
+explain extended (SELECT x FROM t1 FOR UPDATE) LIMIT 1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	100.00	
+Warnings:
+Note	1003	(select `test`.`t1`.`x` AS `x` from `test`.`t1` limit 1)
+drop table t1;
+#
+# End of 10.6 tests
+#