refactor(threads): rename Threads -> Scheduler

[elenaf9]

Description

Rename the central Threads/ THREADS structure in riot_rs_threads into Scheduler/ SCHEDULER.

IMO that's more intuitive given that the structure not just contains and implements the TCBs, but instead all scheduling-related logic. And threads itself is again a property on this struct, so it prevents confusion there (and having to write sth like threads.threads).

Open Questions

Any objections?

Change checklist

• I have cleaned up my commit history and squashed fixup commits.
• I have followed the Coding Conventions.
• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.