Update the crate to compile with newest nightly

[bugadani]

The latest nightly removed the asm! syntax, which broke the ancient cortex-m this crate depends on. This PR updates the dependency and works around the fact that Mutex::lock was somewhere lost in history.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @korken89 (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[diondokter]

Oh whoops, I didn't even look to see if there already was a PR.
I made one too (#34), but mine uses refcell instead of unsafe. So I'm gonna leave mine up so the maintainers can decide which they rather have.

[jonas-schievink]

Merged #26 instead, but thanks anyways!