Implementing cursor%ROWTYPE variables, according to the task description.

This patch includes a refactoring in how sp_instr_cpush and sp_instr_copen
work. This is needed to implement MDEV-10598 later easier, to allow variable
declarations go after cursor declarations (which is currently not allowed).

Before this patch, sp_instr_cpush worked as a Query_arena associated with
the cursor. sp_instr_copen::execute() switched to the sp_instr_cpush's
Query_arena when executing the cursor SELECT statement.

Now the Query_arena associated with the cursor is stored inside an instance
of a new class sp_lex_cursor (a LEX descendand) that contains the cursor SELECT
statement.

This simplifies the implementation, because:
- It's easier to follow the code when everything related to execution
  of the cursor SELECT statement is stored inside the same sp_lex_cursor
  object (rather than distributed between LEX and sp_instr_cpush).
- It's easier to link an sp_instr_cursor_copy_struct to
  sp_lex_cursor rather than to sp_instr_cpush.
- Also, it allows to perform sp_instr_cursor_copy_struct::exec_core()
  without having a pointer to sp_instr_cpush, using a pointer to sp_lex_cursor
  instead. This will be important for MDEV-10598, because sp_instr_cpush will
  happen *after* sp_instr_cursor_copy_struct.

After MDEV-10598 is done, this declaration:

DECLARE
  CURSOR cur IS SELECT * FROM t1;
  rec cur%ROWTYPE;
BEGIN
  OPEN cur;
  FETCH cur INTO rec;
  CLOSE cur;
END;

will generate about this code:

+-----+--------------------------+
| Pos | Instruction              |
+-----+--------------------------+
|   0 | cursor_copy_struct rec@0 | Points to sp_cursor_lex through m_lex_keeper
|   1 | set rec@0 NULL           |
|   2 | cpush cur@0              | Points to sp_cursor_lex through m_lex_keeper
|   3 | copen cur@0              | Points to sp_cursor_lex through m_cursor
|   4 | cfetch cur@0 rec@0       |
|   5 | cclose cur@0             |
|   6 | cpop 1                   |
+-----+--------------------------+

Notice, "cursor_copy_struct" and "set" will go before "cpush".
Instructions at positions 0, 2, 3 point to the same sp_cursor_lex instance.