Change Lex_field_type_st::m_type from enum_field_types to Type_handler pointer

We'll replace the member in Lex_field_type_st::m_type responsible for storing the real data type from:

  enum_field_types m_type; 

to

  const Type_handler *m_handler;

and do relevant changes in sql_yacc.yy and sql_yacc_ora.yy, e.g. replace things like:

  $$.set(MYSQL_TYPE_FLOAT, $2);

to

  $$.set(&type_handler_float, $2);