Add ability to set priority grouping.

[reitermarkus]

Also update interrupt::free to pass CriticalSection<'_> instead of a reference.

[rust-highfive]

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

Please see the contribution instructions for more information.

[reitermarkus]

@adamgreig, I extracted the unrelated changes from this PR. Should be easier to review now.

[reitermarkus]

@adamgreig, ping.