Add a uniffi-bindgen binary to one of the crates.

[afranchuk]

This would be a convenient thing for users to cargo install. I suppose it should be in uniffi since that's where uniffi/cli code exists.

This should come with a warning that the version of uniffi-bindgen used must match the version being used in your crate (and recommend using cargo install --version <version> to guarantee this). This also allows nightly users to easily install the appropriate uniffi-bindgen binary due to this unstable functionality.