[$] Development tools: Sashiko, b4 review, and API specification

The kernel project has a unique approach to tooling that avoids many commonly used development systems that do not fit the community's scale and ways of working. Another way of looking at the situation is that the kernel project has often under-invested in tooling, and sometimes seems bent on doing things the hard way. In recent times, though, the amount of effort that has gone into development tools for the kernel has increased, with some interesting results. Recent developments in this area include the Sashiko code-review system, a patch-review manager built into b4, and a new attempt at a framework for the specification and verification of kernel APIs.