Detect dependencies that are in Cargo.toml but not used in the crate

idea from https://twitter.com/eddyb_r/status/1082458713640841221

Basically detect discrepancies between the `--extern` flags and everything being loaded by the current crate via `extern crate` or implicitly in Rust 2018

related: https://github.com/rust-lang/rust-clippy/issues/1892