Use SocketAddr and `thiserror` errors

[nyurik]

• Socket addr is a more flexible way to store what the mock is bound to, and port can be extracted from that.
• Use thiserror to handle all the error reporting, including the is_fatal() stuff
• I made the code difference between tcp and udp minimal - this way we can eventually consolidate the two variants into one

P.S. a good rule of thumb I found useful for naming variables: the smaller the scope of the variable, the shorter should be its name.

[thomasarmel]

the smaller the scope of the variable, the shorter should be its name. ok

[nyurik]

thx for looking at it! Let me clarify how these levels work, to prevent accidental misuse:

• warnings do not fail compilation - which means it is possible to develop and test things until the PR is ready. This is very important when doing the development because you do not want to be adding comments right away - you don't know at that time if what you are doing is even right or not - so the quicker you can test, the more you can focus on development, the better the result will be.
• CI on the other hand does fail on warnings because of the RUSTFLAGS='-D warnings' - so you won't be able to merge PRs until CI is happy with it.

This achieves what you want -- clean code with comments, while it helps keep developing much faster.

• Lastly, forbid means there can never be any exceptions, no matter how reasonable. deny means it will fail compilation, but in the code it is ok to explicitly allow it when it makes sense. The errors are a good example of that - by design, they already include description as part of the #[error(...)] attribute.
• Note btw that the /// comment is actually also an attribute -- #[doc("comment")] -- so error just a slightly different syntax for the error cases -- if you look at clap crate, you will see that it uses the same approach, only it uses the doc attribute, i.e. the ///
• To sum up: this is a good case for the 3rd coding principle - DRY -- if the error message is already what you want it to be, no point in duplicating that string :)

[thomasarmel]

Ok good for me, thanks!