Support no_std (Rust 1.64 and later) (Take 2)

[chrysn]

Closes: https://github.com/upsuper/cstr/issues/12 (see previous discussion there) Replaces: https://github.com/upsuper/cstr/pull/13

Based on the discussion in #13 I've revisited some assumptions, and removed parts of the PR that were based on my wrong understanding of proc-macro crates.

The patches are now in a sequence that, by running CI on them, shows that the test for no-std behavior alone fails, whereas the second patch fixes it.

As before, this breaks support for Rust versions < 1.64, so CI will report all but the "Build on no_std" tests as failing -- so this only makes sense to merge at the point when 1.64 is stable (at which the "nightly" in the test can be switched over).

[chrysn]

Relevant build reports, now that I've made the github actions happy:

• When only the new test is added, https://github.com/chrysn-pull-requests/cstr/runs/7600662935?check_suite_focus=true#step:4:18 shows how running in a no_std environment fails.
• After std is switched to core, https://github.com/chrysn-pull-requests/cstr/runs/7600665038?check_suite_focus=true shows how the same test passes.

(But of course everything else is not yet on 1.64, so several other tests fail).

[chrysn]

It's certainly nice to have ready-made actions, but I'm not sure this will save more than the rustup target add step -- the dedicated test crate will still be necessary, and only that can be built on the thumb target.

[upsuper]

No, because trybuild already generates test crate automatically. If it can generate a test crate that uses a specific target, the test would be runnable with plain cargo test, and the only change needed for CI task is to install the needed target.

[chrysn]

Short of patching trybuild (which I don't have any capacity to do right now), is there anything I can do to advance this issue?

[upsuper]

Nothing you need to do. I'll probably just merge it when 1.64 reaches stable.

[chrysn]

With 1.64 released, I've been running a round of CI tests at https://github.com/RIOT-OS/rust-riot-wrappers/pull/10#partial-pull-merging in parallel to local tests -- unsurprisingly, there were no surprises.

I've added one commit updating all mentions of required Rust versions to reflect the strategy indicated in the last comment, and taken the liberty to add a rust-version = "1.64" as that enhances the user experience in the error case.

Would you consider making a cstr release soon after merging this?

[upsuper]

New version published.

[chrysn]

Thanks a bunch!