The debug loop that reads a command and does something is part of the
frontend's main loop, so that it can potentially update, even though
it doesn't actually work for minifb because the command input is a
blocking call. It's also not implemented in the pixels frontend.
At some point I'll make a web frontend.
I wanted to make this a bit more modular, so it's easier in theory to
write external crates that can reuse bits, and selectively compile in
bits, such as adding new systems or new cpu implementations
