- Adding Type_handler::make_table_field() and moving pieces of the code
  from Item::tmp_table_field_from_field_type() to virtual implementations
  for various type handlers.

- Adding a new Type_all_attributes, to access to Item's extended
  attributes, such as decimal_precision() and geometry_type().

- Adding a new class Record_addr, to pass record related information
  to Type_handler methods (ptr, null_ptr and null_bit) as a single structure.
  Note, later it will possibly be extended for BIT-alike field purposes,
  by adding new members (bit_ptr_arg, bit_ofs_arg).

- Moving the code from Field_new_decimal::create_from_item()
  to Type_handler_newdecimal::make_table_field().

- Removing Field_new_decimal() and Field_geom() helper constructor
  variants that were used for temporary field creation.

- Adding Item_field::type_handler(), Field::type_handler() and
  Field_blob::type_handler() to return correct type handlers for
  blob variants, according to Field_blob::packlength.

- Adding Type_handler_blob_common, as a common parent for
  Type_handler_tiny_blob, Type_handler_blob, Type_handler_medium_blob
  and Type_handler_long_blob.

- Implementing Type_handler_blob_common::Item_hybrid_func_fix_attributes().

  It's needed for cases when TEXT variants of different character sets are mixed
  in LEAST, GREATEST, CASE and its abreviations (IF, IFNULL, COALESCE), e.g.:
      CREATE TABLE t1 (
        a TINYTEXT CHARACTER SET latin1,
        b TINYTEXT CHARACTER SET utf8
      );
      CREATE TABLE t2 AS SELECT COALESCE(a,b) FROM t1;
  Type handler aggregation returns TINYTEXT as a common data type
  for the two columns. But as conversion from latin1 to utf8
  happens for "a", the maximum possible length of "a" grows from 255 to 255*3.
  Type_handler_blob_common::Item_hybrid_func_fix_attributes() makes sure
  to update the blob type handler according to max_length.

- Adding Type_handler::blob_type_handler(uint max_octet_length).

- Adding a few m_type_aggregator_for_result.add() pairs, because
  now Item_xxx::type_handler() can return pointers to type_handler_tiny_blob,
  type_handler_blob, type_handler_medium_blob, type_handler_long_blob.
  Before the patch only type_handler_blob was possible result of type_handler().

- Making type_handler_tiny_blob, type_handler_blob, type_handler_medium_blob,
  type_handler_long_blob public.

- Removing the condition in Item_sum_avg::create_tmp_field()
  checking Item_sum_avg::result_type() against DECIMAL_RESULT.
  Now both REAL_RESULT and DECIMAL_RESULT are symmetrically handled
  by tmp_table_field_from_field_type().

- Removing Item_geometry_func::create_field_for_create_select(),
  as the inherited version perfectly works.

- Fixing Item_func_as_wkb::field_type() to return MYSQL_TYPE_LONG_BLOB
  rather than MYSQL_TYPE_BLOB. It's needed to make sure that
  tmp_table_field_from_field_type() creates a LONGBLOB field for AsWKB().

- Fixing Item_func_as_wkt::fix_length_and_dec() to set max_length to
  UINT32_MAX rather than MAX_BLOB_WIDTH, to make sure that
  tmp_table_field_from_field_type() creates a LONGTEXT field for AsWKT().

- Removing Item_func_set_user_var::create_field_for_create_select(),
  as the inherited version works fine.

- Adding Item_func_get_user_var::create_field_for_create_select() to
  make sure that "CREATE TABLE t1 AS SELECT @string_user variable"
  always creates a field of LONGTEXT/LONGBLOB type.

- Item_func_ifnull::create_field_for_create_select()
  behavior has changed. Before the patch it passed set_blob_packflag=false,
  which meant to create LONGBLOB for all blob variants.
  Now it takes into account max_length, which gives better column
  data types for:
    CREATE TABLE t2 AS SELECT IFNULL(blob_column1, blob_column2) FROM t1;

- Fixing Item_func_nullif::fix_length_and_dec() to use
  set_handler(args[2]->type_handler()) instead of
  set_handler_by_field_type(args[2]->field_type()).
  This is needed to distinguish between BLOB variants.

- Implementing Item_blob::type_handler(), to make sure to create
  proper BLOB field variant, according to max_length, for queries like:
    CREATE TABLE t1 AS
      SELECT some_blob_field FROM INFORMATION_SCHEMA.SOME_TABLE;

- Fixing Item_field::real_type_handler() to make sure that
  the code aggregating fields for UNION gets a proper BLOB
  variant type handler from fields.

- Adding a special code into Item_type_holder::make_field_by_type(),
  to make sure that after aggregating field types it also properly
  takes into account max_length when mixing TEXT variants of different
  character sets and chooses a proper TEXT variant:
      CREATE TABLE t1 (
        a TINYTEXT CHARACTER SET latin1,
        b TINYTEXT CHARACTER SET utf8
      );
      CREATE TABLE t2 AS SELECT a FROM t1 UNION SELECT b FROM t1;

- Adding tests, for better coverage of IFNULL, NULLIF, UNION.

- The fact that tmp_table_field_from_field_type() now takes
  into account BLOB variants (instead of always creating LONGBLOB),
  tests results for WEIGHT_STRING() and NULLIF() and UNION
  have become more precise.