Path and module changes (RFC 2126)

[matthewjasper]

See rust-lang/rust#44660.

• Paths starting with crate
• Not needing extern crate
• crate visibility modifier - not stabilizing in initial release
• Not using mod.rs
• Any edition lints
• Update existing examples

[ehuss]

I have a branch for the extern crate changes, but it is blocked waiting on #415 (I either need to link to or write something that explains the extern prelude). crate visibility is not stable (rust-lang/rust#53120) so I don't think that's relevant here. I opened PR #450 for mod.rs changes.

I'm not sure what is meant by "edition lints". My extern crate changes note that it is unidiomatic in 2018, but I'm not sure what other lints are relevant. Also, I was under the impression lints aren't generally documented here and deferred to the rustc user guide. Are there specific rust_2018_idioms you want to call out?

[ehuss]

Regarding updating the examples, AFAIK the mdbook playpen links don't support changing the edition, so those links will always break. I can't find this being tracked or mentioned anywhere. I imagine they aren't used very often, but I think it should be tracked somewhere (should I file an mdbook issue?).