Add convenience trait impls for CName

[Roms1383]

2 simple traits impl for CName :

• AsRef<str>
• ToString

Concerning safety, I assumed from resolve_cname signature that AsRef<str> is safe since it returns a &'static str, but let me know if there's any concern with it. Otherwise having simply ToString will already improve ergonomics a bit :)

[jac3km4]

I wouldn't implement AsRef<str> here because it's meant to be used for simple reference to reference conversions and this involves doing a lookup in a pool managed by the game.
As for ToString, I think it's okay, although it's probably a better idea to implement fmt::Display, since ToString is automatically implemented for types with fmt::Display

[Roms1383]

@jac3km4 I also added #[cfg(not(test))], to prevent people from inadvertently using Display in their unit-tests, since resolve_cname can only be used in-game. Let me know if there's a better directive.