Run cargo udeps as part of CI

[D4nte]

Problem

We have no way to ensure that any of the deps in Cargo.toml are used.

Goal

Make CI fail if Cargo.toml contains unused deps

Recommendation

Try with https://lib.rs/crates/cargo-udeps

[thomaseizinger]

We may want to consider not running it as part of CI because the README of this thing says it can yield false-positives.
I'd be keen to have a GitHub action that runs every day or something and opens a PR that removes the resulting dependency instead of doing it as part of CI. This way we would have more control over false-positives and can add them to a manual ignore list or something.
See actions-rs/meta#11.

[D4nte]

Udeps is not stable:

error: internal compiler error: src/librustc/ty/context.rs:211: node type <L>::Identity (hir_id=HirId { owner: DefIndex(4112), local_id: 5 }) with HirId::owner DefId(0:4112 ~ cnd[3821]::http_api[0]::routes[0]::rfc003[0]::accept[0]::_IMPL_DESERIALIZE_FOR_OnlyRedeem[0]::{{impl}}[0]) cannot be placed in TypeckTables with local_id_root DefId(0:4109 ~ cnd[3821]::http_api[0]::routes[0]::rfc003[0]::accept[0]::_IMPL_DESERIALIZE_FOR_OnlyRedeem[0])

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:917:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.40.0-nightly (20cc75272 2019-10-09) running on x86_64-apple-darwin

note: compiler flags: -Z save-analysis -C debuginfo=2 -C incremental --crate-type lib

note: some of the compiler flags provided by cargo are hidden

error: aborting due to previous error

Error: StrErr("Could not compile `cnd`.")