By Alun John HONG KONG (Reuters) -TerraUSD, one of the world’s largest stablecoins, lost a third of its value on Tuesday, spooking cryptocurrency investors and partly contributing to bitcoin’s tumble below $30,000 for the first time in 10 months. Stablecoins are digital tokens pegged to the value of traditional assets, such as the U.S. dollar. They are popular as safe havens in times of turmoil in crypto markets and are a common medium of exchange, often used by traders to move funds around and speculate on other cryptocurrencies. TerraUSD, also known as ‘UST’, is a so-called algorithmic stabl…