Cannes: Socially relevant gloom is not the sole mode of "Ashes," a tight yet potent narrative that addresses migration, abandonment, and the constant sense of displacement that plagues immigrants. Something no one tells you about leaving your homeland is that, just like you can never step into the same river twice, returning doesn’t mean belonging again. Halfway through Diego Luna ’s latest effort as a director, “ Ashes ” (“Cenizas en la boca”), an understandably angsty teenager, Diego (Sergio