Prepare for 2018 edition

[Xanewok]

While dyn fixing annotations was straightforward, doing that for urneachable_pub was awkward... Not sure I agree with most of the changes - IMO pub(crate) adds more noise and regular pub should be treated as such, especially for bin targets, but that may be just me. Tried to go for the most restrictive visibility for the warned against items (not sure if that's good).

r? @nrc

[nrc]

Thanks for doing this! I think I would change all the pub(super) to pub(crate), and you can use crate rather than pub(crate) to make it a bit less noisey.

Do we need to opt-in to the edition in Cargo.toml?

[Xanewok]

Used crate and now the edition is in the Cargo.toml, thanks!

Did use official Edition transition guide (+ added #![warn(rust_2018_idioms)]) and rustfix for this time around.

So the experience with using cargo fix was somewhat good, although it did just mostly slap crate:: to imports.
The main problems I had:

1. No obvious way to use it for other targets (i.e. tests) and --cfged code (Used it manually in the end)
2. It just automatically applies rustc suggestions, which in this case wasn't enough - here we extern crate rls_span as span in main.rs file and use span; in other, but it couldn't handle these reexported extern crates.

For extern rls_* crates I didn't pub use them in main to be later reused as crate::<name> elsewhere, since I believe making the distinction makes it easier to build a mental model of the RLS and helps discovery of those external crates (I know I was confused at the beginning).

[nrc]

No obvious way to use it for other targets (i.e. tests) and --cfged code (Used it manually in the end)

--all-targets should work for this.

Thanks for the feedback!