generators::legacy0 partial implementation.
generators::legacy0 integration with mysql.
generators::legacy0::generate(): statement support + refactoring.
generators::legacy0::generate(): problem_{ml,tl} bug fix.
generators::legacy0::generate(): SQL bug fix.
generators::legacy0::generate(): SQL encoding bug fix.
generators::legacy0 refactoring.

MySQL connection object became class member.
generators::legacy0 refactoring.

New class: db_connection.
db_connection cache implementation.