Refactor: Course state file should contain path as id and not indexes

[samuelroland]

Currently, the .course-state.toml at repos root is serialized with the given fields. The problem is that using indexes might cause issues when new exos or skills are created, therefore shifting the indexes... When resuming an exo it might jump to an older exo.

pub(crate) curr_skill_idx: usize,
pub(crate) curr_exo_idx: usize,

A more stable identifier would be to store the path.