cargo library interface is unstable

[laudiacay]

The Cargo library is not guaranteed to have any stable API at all- you're supposed to shell out to the cargo binary instead.

Things will continue to break periodically until we fix this- if we don't update the Cargo library version, some Cargo.toml files will break siderophile, and if we do update, things will just break. So at some point I need to change this to call the binary instead.