More deny directives should be added

[Taowyoo]

According to @TylerBloom

After stabilization (i.e. during the 3.5.0), more deny directives should be added. IMO, a decent starting list is:

#![deny(
    unused,
    dead_code,
    missing_docs,
    missing_debug_implementations,
    rustdoc::broken_intra_doc_links,
    unreachable_pub,
    unused_allocation,
    unused_comparisons,
    unused_parens,
    while_true,
    trivial_casts,
    trivial_numeric_casts,
    unused_extern_crates,
    unused_import_braces,
    unused_qualifications,
    unused_results,
    clippy::all,
)]
#![warn(rust_2018_idioms)]