[$] A pair of Rust kernel modules

The idea of being able to write kernel code in the Rust language has a
certain appeal, but it is hard to judge how well that would actually work
in the absence of examples to look at. Those examples, especially for
modules beyond the "hello world" level of complexity, have been somewhat
scarce, but that is beginning to change. At the 2022 Kangrejos gathering
in Oviedo, Spain, two developers presented the modules they have developed
and some lessons that have been learned from this exercise.