“Everyone has a story about the Krays,” says an off-screen narrator in the first moments of “Legend.” Then she adds, “They were gangster princes.”They were the identical twins Reggie and Ron Kray, major components of the London crime scene in the 1960s. They’re both played, under the guidance of writer-director Brian Helgeland (“42,” “Payback”), by Tom Hardy, much in the same way David Cronenberg directed Jeremy [...]