WASHINGTON, Feb 4 (Reuters) - President Donald Trump's overhaul of U.S. foreign assistance has led to chaos in the aid and development field, leaving hundreds of contractors in a severe financial crunch with some already having to lay off staff and others facing millions of dollars in unpaid invoices. Hours after coming into office on Jan. 20, Trump ordered a sweeping review of almost all U.S. foreign aid and tasked billionaire Elon Musk, who has falsely accused USAID of being a "criminal" organization, with scaling down the agency. Since then, dozens of USAID staff has been put on leave, hundreds...