Just about the biggest bloodletting in the current series of computer industry retrenchements has come at Wang Laboratories Inc, and the Lowell, Massachusetts company is not finished yet. From 31,000 in March 1989, it is now down to 19,000 employees, and another 500 are to go in the next three weeks. The company says that while its internal operations are back in balance and its bank debt is paid off, it is now having to adjust to recession.