Make sleep/1 interruptible by Ctrl-C

Make sleep/1 interruptible by Ctrl-C

sleep/1 previously called a single blocking std::thread::sleep for the
whole duration, which runs to completion regardless of SIGINT. The
interrupt flag was only polled by the dispatch loop every 256
instructions, so a sleep-dominated loop kept running for a
nondeterministic number of extra iterations after Ctrl-C.

Sleep in 50ms slices and poll interrupt_occured() between them, throwing
the interrupt exception promptly when the flag is set.