feat(threads): sort ThreadList by priority

[elenaf9]

Description

Insert new threads in ThreadList based on priorities, so that the highest priority thread is the head.
Within a priority the threads are ordered in FIFO.

This reduces priority inversions, and will facilitate proper priority inheritance in the future.

The drawback of this change is that inserting a new Id in the ThreadList is now more complex. Not sure if we can avoid this in any way.

Issues/PRs references

Alternative to #371, which implements the ThreadList as pure FIFO.

Open Questions

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.

[kaspar030]

Nice! Changes to riot-rs-threads look fine.

Can we turn the example into a test (in tests/), and add some logic after each acquire() that e.g., checks the value of an atomic uint if it contains the right value, so we can test the order programatically? (e.g., first thread expects a zero, then sets bit zero. second expects bit zero, then ORs in bit one. third one expects bits zero and one, ...)

If you feel changing that now is out of scope, I'd suggest dropping the example and we add tests later.

[elenaf9]

Can we turn the example into a test (in tests/), and add some logic after each acquire() that e.g., checks the value of an atomic uint if it contains the right value, so we can test the order programatically? (e.g., first thread expects a zero, then sets bit zero. second expects bit zero, then ORs in bit one. third one expects bits zero and one, ...)

Wdyt of 67a8e9c?

[kaspar030]

LGTM, please squash!