LITTLE ROCK, Ark. — A federal appeals court struck down one of the nation’s toughest abortion restrictions on Wednesday, agreeing with a lower court that ruled an Arkansas law unconstitutionally burdens women by banning abortions after the 12th week of pregnancy if a doctor can detect a fetal heartbeat.The 8th U.S. Circuit Court of Appeals sided with doctors who challenged the law, ruling that abortion restrictions must be based on a fetus’ ability to live outside the womb, not the presence of a fetal …