Migrate to Rust 2018

[taiki-e]

Since #69, the minimum Rust version supported by this crate is 1.36 (because nix 0.18 use std::mem::MaybeUninit that stabilized in 1.36), so we can migrate to the 2018 edition without breakage.

(This patch was created primarily by running cargo fix --editon, removing needless extern crate, and removing self:: prefix from external crate paths.)

[taiki-e]

CI failure seems unrelated:
https://ci.appveyor.com/project/Detegr/rust-ctrlc/builds/36213138/job/n7075os1fgg4ukqa:

warning: spurious network error (2 tries remaining): error inflating zlib stream; class=Zlib (5)
warning: spurious network error (1 tries remaining): error inflating zlib stream; class=Zlib (5)
error: failed to get `nix` as a dependency of package `ctrlc v3.1.7 (C:\projects\rust-ctrlc)`

https://ci.appveyor.com/project/Detegr/rust-ctrlc/builds/36213138/job/35bvkdof3savoyf7:

warning: spurious network error (2 tries remaining): error inflating zlib stream; class=Zlib (5)
warning: spurious network error (1 tries remaining): error inflating zlib stream; class=Zlib (5)
error: failed to get `nix` as a dependency of package `ctrlc v3.1.7 (C:\projects\rust-ctrlc)`

[Detegr]

Thank you. Excellent explanation of how this should be safe to do without breakages.