Prepare cargo update to cargo 0.56

Preperation patch for updating to Cargo 0.56. This has all the bit needed up to actually importing the upstream orig and vendored tarball. Some relevant patches cherry-picked and fixed from !8 (closed) also MR against experiment so some patches in the MR are from the debian/sid branch.

Full import done in my repository as an poc for the next step (debian/experimental branch).