A benchmark on the trivial cgo call, to assess overhead of different

ways of calling out to cgo.

usb/libusb.go

@@ -381,3 +381,8 @@ func xfer_callback(cptr unsafe.Pointer) {
 	ch := *(*chan struct{})(cptr)
 	close(ch)
 }
+
+// for benchmarking
+func libusbSetDebug(c *libusbContext, lvl int) {
+	C.libusb_set_debug((*C.libusb_context)(c), C.int(lvl))
+}