Make Rust upgradable

[brson]

Once we have #9877, we also need to be able to upgrade seamlessly from one release (or snapshot) to the next. This means having an upgrade strategy for both the stuff we install to the system directories and libraries that rustpkg leaves sitting around. Once we hit 1.0 it's not ok to just expect users to figure it out and clean up the stale files we leave sitting around.

[brson]

Nominating

[brson]

With #13524 the core Rust distribution is upgradable on all plattforms. The rest is up to cargo.