By Alun John and Hannah Lang LONDON (Reuters) -Crypto exchange FTX filed for U.S. bankruptcy proceedings on Friday and founder Sam Bankman-Fried stepped down as CEO, in a stunning downfall that has sent shock waves through markets and drawn calls for better regulation of the digital industry. The distressed crypto trading platform had been struggling to raise billions in funds to stave off collapse after traders rushed to withdraw $6 billion from the platform in just 72 hours and rival exchange Binance abandoned a proposed rescue deal. The company said in a statement shared on Twitter on Frida…