feat: Implement hash for Code

[faern]

Motivation

I want to have a HashMap<Code, u64> to keep some statistics on various codes servers return. But Code is not Hash. I don't see any reason why it would not be. It's just a simple enum.

I can of course keep a HashMap<u32, u64> and upon insertion cast the enum to an integer. But I don't know the width of the integer as it's not part of the public API. And it also does not allow me to get the nice string description of the code out of the hashmap keys later.

Solution

Implement Hash for Code.