By Tom Wilson LONDON (Reuters) – Bitcoin tumbled on Wednesday to a new 18-month low, dragging smaller tokens down with it and deepening a market meltdown sparked by crypto lender Celsius this week freezing customer withdrawals. The world’s largest cryptocurrency fell as much as 7.8% to $20,289, its lowest since December 2020. It has lost around 28% since Friday and more than half of its value this year. It has slumped about 70% from its record high of $69,000 in November. The digital currency sector has been pummelled this week after U.S. crypto lender Celsius froze withdrawals and transfers b…