Support server fuzzing

[Aaron1011]

This PR makes several changes to allow fuzzing of a Roughtime server in roughenough-fuzz:

• Extracts server code from roughenough-server.rs into a new Server struct.
• Moves the core server processing into a standalone function, suitable for calling from a fuzz wrapper.
• Adds a MemoryConfig struct, to allow a fuzz wrapper to easily create a ServerConfig.
• Adds a new method Server::send_to_self, gated behind #[cfg(fuzzing)]. It allows the server to feed messages to itself using a separate client UdpSocket instance.
• Add a new --outputflag to roughenough-client, which causes the client to write all of its requests to a file. This can be used to easily generate fuzzer inputs to the server.

I've left this PR as a series of several commits, but I can squash them once you think this is ready to merge.

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[int08h]

Hi @Aaron1011! This is really awesome.

I'm very impressed you based this on 1.1 as I'm churning on that a lot, sorry if it caused you extra work.

I'm going to go ahead and merge this to 1.1 so you can keep going. I think a errant hyper reference broke the build. ~I'm not sure if I introduced it or one of your changes.~ Nope, I broke it in 54955e4. Fixing it now.

[int08h]

@Aaron1011 thanks for your continued contributions to Roughenough. I've added you as a contributor to roughenough and roughenough-fuzz so you can commit directly. You're the first roughenough contributor other than myself and I honestly never expected to attract much interest!

I don't have any grand "governance" plans. For now, how about we mutually coordinate future releases? Additionally, given your more extensive experience as an open-source contributor if you have suggestions on how we best coordinate a small project like this, I'm open to them.