Consider moving to xtask

[Kixunil]

xtask pattern could improve compilation times for libraries. However it's not entirely suited for this kind of optimization. So instead we can put codegen into cfg_me and if it's installed just run it, if it's not run xtask which will do the same thing.

And make sure cfg_me is versioned similarly to cargo +version ... (but maybe scan the project file for version instead?) so that we can make sure codegen matches.