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.

Developer Certificate of Origin
Version 1.1

Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
1 Letterman Drive
Suite D4700
San Francisco, CA, 94129

Everyone is permitted to copy and distribute verbatim copies of this
license document, but changing it is not allowed.

Developer's Certificate of Origin 1.1

By making a contribution to this project, I certify that:

(a) The contribution was created in whole or in part by me and I
    have the right to submit it under the open source license
    indicated in the file; or

(b) The contribution is based upon previous work that, to the best
    of my knowledge, is covered under an appropriate open source
    license and I have the right under that license to submit that
    work with modifications, whether created in whole or in part
    by me, under the same open source license (unless I am
    permitted to submit under a different license), as indicated
    in the file; or

(c) The contribution was provided directly to me by some other
    person who certified (a), (b) or (c) and I have not modified
    it.

(d) I understand and agree that this project and the contribution
    are public and that a record of the contribution (including all
    personal information I submit with it, including my sign-off) is
    maintained indefinitely and may be redistributed consistent with
    this project or the open source license(s) involved.

[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.