Add Field_row for SP ROW variables

We'll solve MDEV-13418 by translating:

SELECT  1, a INTO a, b FROM dual;

into

DECLARE a_tmp TYPE OF a;

DECLARE b_tmp TYPE OF b;

SELECT  1, a INTO a, b FROM dual;

SET a=a_tmp;

SET b=b_tmp;

Notice, this will need auto-generated assignments between SP variables (a=a_tmp and b=b_tmp).
Normally assignment is done via sp_instr_set, but it does not suite well here because it needs:

• an Item for the assignment source
• a LEX containing this Item

Instead of creating an Item and a LEX (whose sizeof are 208 and 5896 bytes respectively),
we'll introduce a new sp_instr_setvar which will copy between two variables directly, with no Item and LEX, using their offsets in sp_rcontext::m_var_table.

In order to implement this, we'll need some changes for ROW-type SP variables.
Currently ROW variables store their members inside Item_splocal_row::m_table.
We'll do the following:

• Create a new class Field_row and move m_table from Item_splocal_row to Field_row

  class Field_row: public Field_null

  {

  class Virtual_tmp_table *m_table;

  public:

  Field_row(uchar *ptr_arg, const LEX_CSTRING *field_name_arg)

  :Field_null(ptr_arg, 0, Field::NONE, field_name_arg, &my_charset_bin),

  m_table(NULL)

  {}

  };

• Remove the class Item_splocal_row and use Item_splocal instead by passing &type_handler_row to the constructor.

• Change Item_splocal::Item_splocal to accept const Type_handler *handler instead of enum_field_types sp_var_type.

• Remove the class Item_spvar_args and derive Item_field_row directly from Item_args

• Move row_create_items() and get_row_field() from Item_spvar_args to Item_field_row and fix them to use Field_row::m_table instead of Item_spvar_args::m_table.

• Replace DBUG_ASSERT in Type_handler_row::Column_definition_fix_attributes(), Type_handler_row::Column_definition_prepare_stage1(), Type_handler_row::Column_definition_prepare_stage2() to real appropriate implementations.