Function resolve_env_string 

pub fn resolve_env_string(aliases: &[EnvAlias]) -> Option<String>

Resolve an env-var string through the alias list. The first alias whose lifeloop key is set wins; otherwise the first alias whose ccd_compat key is set wins. Whenever an alias has both sides set, a precedence warning is recorded (once per lifeloop key per process) regardless of which alias actually carried the resolution.