Support for projects not using `cargo`

[emarteca]

Currently, the library_mode generate_bindings calls cargo to retrieve metadata about the project for which bindings are being generated: e.g. generate_bindings starts with this line:

let cargo_metadata = MetadataCommand::new()
        .exec()
        .context("error running cargo metadata")?;

This means uniffi-rs does not work for projects that use a different build system, where cargo is not available and/or where the project is missing a Cargo.toml file.

It would be nice if uniffi-rs was generalized to not be reliant on cargo.