“America’s practice of mass incarceration is coming under growing criticism as fiscally unsustainable and morally indefensible. Chronic overcrowding of prisons, a problem that epitomizes the destructive and unlawful core of mass incarceration, now afflicts the federal prison system and nearly half the states. Actual reforms, however, like President Obama’s recent grant of clemency to forty-six federal prisoners serving long drug sentences for non-violent conduct, or recent one-off sentencing reforms aimed at preventing imprisonment for minor drug or property crimes, are manifestly insufficient to end mass incarceration, or even the chronic overcrowding that represents its most degrading and destructive aspect.”