Example: Dining cryptographers

[dvdplm]

Implement Dining cryptographers problem in manul, as an example of a non-trivial protocol.

• Implement the example
• Random documentation tweaks outside the example proper
• A "guide" level document thought to be an intro to manul. Probably needs more/better material but it's a start.
• Where should it live?
• Should it perhaps be a part of the crate docs?
• Should I remove it and make into a separate PR?

Questions

• There are plenty of questions in the code
• How should we run examples? In this PR I suggest using [[bin]] in the manul-examples cargo file so users can run with cargo run, but open to other suggestions.
• Does this example complement the existing ones or does it replace them (in part, in full)?
• Do we need more examples or is this enough for now?

[fjarri]

The code looks good to me, but I'm still not convinced about the directory layout. It looks weird, especially next to the other example which is a library. Are you sure this makes it easier to understand as opposed to it being in src/? It would still be a single file, just main() would be a test.

[fjarri]

Just realized there's #115. I guess this'll go in then, but I do hope we clean it up soon (famous last words, I know).