Switch to the new Cargo.lock format

This was achieved by deleting Cargo.lock
and letting a recent Cargo nightly re-create
it. Support for the format was already
introduced in Rust 1.38, but currently,
stable releases of Cargo only retain it
if encountered but don't generate such
files by default.

The new format is smaller, better suited to
prevent merge conflicts and generates smaller
diffs at dependency updates, leading to
smaller git history.