UsbContext trait did not require implementers to be Send and Sync.

Affected versions of rusb did not require UsbContext to implement Send
and Sync. However, through Device and DeviceHandle it is possible to use
UsbContexts across threads.

This issue allows non-thread safe UsbContext types to be used concurrently
leading to data races and memory corruption.

The issue was fixed by adding Send and Sync bounds to UsbContext.