Improve types-related helper methods of TableMapEvent

[IANTHEREAL]

@huangjunwen contributed a great PR that enhances MySQL-8.0 binlog replication #468

Because of the complexity of the MySQL type system, go-mysql needs interested/senior guys to help with types-related helper methods in TableMapEvent :|

[huangjunwen]

Hey, today i found a method Table_map_log_event::print_columns (in mysql-8/sql/log_event.cc or mariadb-10.5/sql/log_event_client.cc) which i didn't notice before. Which is added to support mysqlbinlog --print-table-metadata. I think we can implement these helper methods base on this.

I've made some modification on my fork, looks good so far.