Move Field_geom from field.cc to sql_type_geom.cc

To turn the implementation of the GIS data types into a plugin easier, let move Field_geom from field.h and field.cc to sql_type_geom.h and sql_type_geom.cc.

Let's also cleanup the code in the way that Field_geom is used only in

• sql_type_geom.h - the class definition
• sql_type_geom.cc - the class implementation, and use cases from Type_handler_geometry
• item_geofunc.cc - in Item_func_spatial_rel::get_mm_leaf()

Field_geom is currently used in storage/mroonga/ha_mroonga.cpp, in cast targets. All casts targets should be changed to Field_blob, as the rest of the code uses nothing Field_geom specific.

Field_geom is also used in handler.h, in this method:

 virtual bool can_convert_geom(const Field_geom *field,

                               const Column_definition &new_type) const

{

  return false;

}

Note, this method is not overloaded by any engines at the moment. So it can be removed for now. When engines will want to do instant ALTER for GEOMETRY (for example from POINT to GEOMETRY), we can restore this method, but using some generic pointer type instead of Field_geom.