Add class Type_collection

Let's add a new class with approximately this definition:

class Type_collection

{

public:

  virtual ~Type_collection() {}

  virtual const Type_handler *aggregate_for_result(const Type_handler *h1,

                                                   const Type_handler *h2)

                                                   const= 0;

  virtual const Type_handler *aggregate_for_comparison(const Type_handler *h1,

                                                       const Type_handler *h2)

                                                       const= 0;

  virtual const Type_handler *aggregate_for_min_max(const Type_handler *h1,

                                                    const Type_handler *h2)

                                                    const= 0;

  virtual const Type_handler *aggregate_for_num_op(const Type_handler *h1,

                                                   const Type_handler *h2)

                                                   const= 0;

};

It will work as follows:

• Data type plugins will provide instances of Type_collection.
• Each Type_collection will contain one or more data types (Type_handler instances).
• Data types from the same Type_collection will "know" each other.
• If two data types are from the same collection, data type aggregation for various operations (i.e. result, comparson, LEAST/GREATES, numeric dyadic operators) will be delegated to their Type_collection through virtual methods.

For now there will be two collections:

• Type_collection_geometry, containing Type_handler_geometry.
• Type_collection_std, containing all other built-in data types.

Later, we'll split out geometry subtypes (POINT, LINESTRING, POLYGON, MULTIPOINT, MULTILINESTRING, MULTIPOLYGON, GEOMETRYCOLLECTION) into separate type handlers, so Type_collection_geometry will contain 8 type handlers
(the generic one, and handlers for 7 subtypes):

• Type_handler_geometry
• Type_handler_point
• Type_handler_linestring
• Type_handler_polygon
• Type_handler_multipoint
• Type_handler_multilinestring
• Type_handler_multipolygon
• Type_handler_geometrycollection

Later, we'll add Type_collection_json and will turn JSON functionality into handlers:

• Type_handler_varchar_json
• Type_handler_tinytext_json
• Type_handler_text_json
• Type_handler_mediumtext_json
• Type_handler_longtext_json
  and remove Item::is_json_type().

Later, we'll add Type_collection_inet with two data type handlers:

• Type_handler_inet4
• Type_handler_inet6