Use workspace-level lints

[newpavlov]

No description provided.

[newpavlov]

@tarcieri
WDYT should we keep #![forbid(unsafe_code)] in each crate or move it to the workspace?

Also note that I removed the doc_markdown lint since it was causing a fair number of false positives.

[tarcieri]

I've moved unsafe_code = "forbid" to Cargo.toml in repos where it makes sense. Also removed doc_markdown in other places, though in some cases it's found legitimate problems.