This is the first testing release for Tock 2.2!

With almost two years since Tock 2.1.1, it's about time for a new Tock release! Compared to 2.1.1, this release will contain 3882 commits made in 836 PRs by 90 contributers (of which 48 are new contributors!). This release is the first to support stable Rust for out-of-tree and (a subset of) the in-tree boards, adds new supported boards, and contains many other fixes and improvements. Some notable bug fixes are in the Cortex-M arch crate to prevent userspace applications from executing in privileged mode, and disabling unused PMP regions on RISC-V. A full change list will be included with the final release notes.

Release organization and testing will commence at https://github.com/tock/tock/issues/4272.