Secretary-General Kofi Annan, after being horrified by findings of widespread child sex abuse by aid workers in West Africa, vowed yesterday to quickly look into the matter and crack down hard on wrongdoers.  A UN team was in the region to check the findings, made public Tuesday, of the study by the UN High Commission on Refugees and Britain's Save the Children.  The study of refugees in Guinea, Liberia and Sierra Leone found that almost 70 aid workers from 40 agencies had been pushing refugee children into sex for food, medicine and other supplies sent to save their lives.