Implement UDS service with example

[Jhynjhiruu]

This PR adds basic local networking functionality with the UDS service. It's very much not feature-complete, but enough is implemented to be able to interface with the devkitPRO local networking example - i.e. joining a network, creating a network, sending and pulling packets, and basic configuration. This is probably enough functionality for most homebrew applications, and I plan to try and implement a Download Play sniffer with it.

Todo: finish implementing the rest of the functions; test properly; cleanup code?

An example is included (local-networking.rs; this is a mostly-but-not-entirely-complete port of the devkitPRO local networking example).

[Jhynjhiruu]

Is there any way I can test things out, maybe with an emulator?

I've looked a little bit into it and I think it's maybe possible to connect to a 3DS's network with another device, but it'd require more knowledge about how Wi-Fi works than I have. I think Citra supports relaying local networking data for multiplayer, but I don't know how to set it up.

[Jhynjhiruu]

Functions left to implement:

• [ ] udsGenerateNodeInfo (not really intended for use standalone, I think, and doesn't really need to be in the safe API)
• [ ] udsGetNodeInfoUsername (the safe API replaces the NodeInfo type and that decodes the string, so there's no need for this)
• [ ] udsCheckNodeInfoInitialized (I have no idea why this is supposed to be useful, since you always know this already)
• [ ] udsGenerateDefaultNetworkStruct (also not really intended for use standalone, used as part of Uds::new())
• [x] udsWaitDataAvailable (would be good to have, but not really needed since you can just check whether Uds::pull_packet() returns Some or None)
• [x] udsEjectClient
• [x] udsEjectSpectator
• [ ] udsUpdateNetworkAttribute (definitely not really intended for use standalone, other functions call this internally)
• [x] udsSetNewConnectionsBlocked
• [x] udsAllowSpectators
• [x] udsGetNodeInformation

So, only about 5½ that would be actually useful, and they're mostly trivial.