Attorney General Maura Healey is pushing a bill that would make trafficking of the powerful synthetic painkiller fentanyl a crime punishable by up to 20 years in prison, her office announced today.Healey was expected to unveil the legislation at an early afternoon press conference with Milford state Rep. John Fernandes, co-chair of the legislature’s judiciary committee, along with a family of an overdose victim and what her office called a “coalition of law enforcement advocates.”