Add a library crate

[fenhl]

It would be useful to be able to use this tool's functionality from Rust code without having to go through the CLI. Is that something that could realistically be added?

[marcospb19]

It's unlikely that we'd move forward with a full "librification" of cargo-sweep, but if a volunteer created a library with equivalent functionality, we could use it here.

The code is licensed under MIT.