a1ien/rusb#213

Inspired by #76, I realized that libusb is a wrapper of OS APIs, and rusb is yet another wrapper around libusb.

If only the authors of rusb and nusb had worked together and made newer versions of rusb on the nusb code base, improvements to the new library would have been much faster, making it much more popular than the current nusb.

Currently, I'm more familiar with rusb instead of asynchronous programming. I'll provide a link to my Android minimal CDC-ACM driver crate (which is unimplemented in Android OS), with the rusb wrapper as its private module. Hopefully someone will do it for nusb, or just guide me to make the port (of course, some parts of the existing linux_usbfs code can be used).