Port capture backend from `rusb` to `nusb`

[martinling]

This PR rewrites the capture backend to use nusb instead of rusb.

Using nusb has the following advantages for us over rusb:

• Eliminates a layer of indirection and a dependency on the libusb C library: nusb uses OS APIs directly.
• Has an API that supports queuing multiple bulk transfers on one endpoint, maximising capture throughput.
• Seems more likely to gain hotplug and RAW_IO support on Windows, both of which are long-stalled in libusb.

This PR includes the first usage of async in Packetry. I have tried to bring in a minimum of baggage with this:

• futures-lite for the block_on function.
• futures-channel for the oneshot channel, to signal the capture task to stop.
• futures-util for the select_biased macro and FutureExt::fuse method.

The latter two crates are already in our dependency tree as they are used by the gio and glib crates.

Tested on Linux with Cynthion r1.4.

[miek]

I just tried this out and I find it gets out of sync on capture start quite often, and gets stuck displaying malformed packets.

[martinling]

@miek I think I've fixed the issues with this now.