[experimental] rust: Update versions

[chrysn]

[updated description; see below the line for the original version]

This PR is a template for what needs to be done when updating the riot-sys and riot-wrappers Rust crates used in the examples. Usual steps are:

• PR the changes to the relevant repository https://github.com/RIOT-OS/rust-riot-wrappers or https://github.com/RIOT-OS/rust-riot-sys.
• Merge them there.
• Create a new branch here from master, and run

for x in examples/rust* tests/rust*; do cd $x; cargo update || break; make BOARD=samd21-xpro CARGO_CHANNEL=nightly || break; cd -; done
git commit */*/Cargo.lock -m "rust: Update versions"

• Push that into a new PR, briefly summarizing why an update is necessary.
• Let Murdock, ACKs and merging take their course.

Apart from being useful as a template for other PRs, this PR is also occasionally used to just run these steps (force pushing, without an intention to merge) to get the latest -sys and -wrappers tested by Murdock.

---

This is a test balloon for how #17488 could turn out.

This is a draft commit because the git repo is not yet one RIOT controls, and anyhow there's no good reason to switch to an unreleased version right now.

[chrysn]

There are now clones of the relevant crates in https://github.com/RIOT-OS-pkgmirror.

By now, this always-experimental PR is not showing how they'd be used (which should be straightforward from swapping around the upstreams), but also to give branches of riot-sys and -wrappers a trial run on the full set of boards rather than the limited set their own CI can do.

[chrysn]

With #18151 merged, this branch can now be generated more easily: Just run

for x in examples/rust* tests/rust*; do cd $x; cargo update || break; make BOARD=samd21-xpro CARGO_CHANNEL=nightly || break; cd -; done
git commit */*/Cargo.lock -m "rust: Update versions"

(which includes a simple local build check just see issues early).

The top description is being updated to reflect how this PR works now.

[chrysn]

This was accidentally marked as merged when the branch happened to be on the same commit as a different PR (that was not experimental/always-open) and the latter was merged. Trying to re-open it...