Replace cargo-generate with kickstart

[eupn]

I think it could be better to replace cargo-generate with a similar but more powerful tool kickstart. This tool supports custom template variables. This should free the user of most of the manual actions of replacing {{variables}} that aren't supported by cargo-generate.

If it makes sense to do so, I'll be happy to make a PR for this.

[jonas-schievink]

I think I'm on board with this. This seems like a straightforward way to improve the experience.

We are also thinking about making a more elaborate embedded-specific project creation tool, but that's definitely a longer-term project, so this seems like a good intermediate step.