Add migration for data

[sosthene-nitrokey]

Replace #8

[sosthene-nitrokey]

I made the migration and the removal of the dat folder optional, so this can be merged without staying a pain having to constantly rebase.

[robin-nitrokey]

I see you tagged this as 0.3.1. Shouldn’t it be a minor release instead because of the breaking changes at least in Se050Backend::new? Though effectively it does not make a big difference because we only use it in the runner.