Update starch to the latest version

[mutability]

This brings dump1090's use of starch up to date with the current version (following starch updates used by libpg2sdr)

• rearrangement of how flavors / features work; the diff looks big but it's mostly just separating out implementations to separate files and some minor macro tweaks.
• removal of alignment variants, which added a lot of complexity for minimal performance difference
• make the starch subdir be a git submodule, rather than a full copy. This submodule is only needed if you're actually regenerating the generated code, it's not needed for normal builds.
• run starch-benchmark -V as part of make test (which is run by the build workflows)