Use edition 2018 everywhere (#636)

Author: MaxSem

CONTRIBUTING.md

@@ -271,9 +271,7 @@ Mark examples that depend on external systems with `no_run` or remove them
 if they are not required for the example.  Avoid inline comments, preferring
 explanation in the description.
 
-> ```rust
-> extern crate rand;
->
+> ```rust,edition2018
 > use rand::distributions::{Normal, Distribution};
 >
 > fn main() {

build.rs

@@ -1,6 +1,3 @@
-extern crate skeptic;
-extern crate walkdir;
-
 use walkdir::WalkDir;
 
 const REMOVED_TESTS: &[&str] = &[

src/concurrency/thread/crossbeam-complex.md

@@ -24,10 +24,7 @@ to prevent the entire program from blocking on the worker for-loops. You can
 think of the calls to `drop` as signaling that no more messages will be sent.
 
 
-```rust
-extern crate crossbeam;
-extern crate crossbeam_channel;
-
+```rust,edition2018
 use std::thread;
 use std::time::Duration;
 use crossbeam_channel::bounded;

src/science/mathematics/linear_algebra/deserialize-matrix.md

@@ -6,10 +6,7 @@ by [`serde_json::to_string`] and [`serde_json::from_str`] performs deserializati
 
 Note that serialization followed by deserialization gives back the original matrix.
 
-```rust
-extern crate nalgebra;
-extern crate serde_json;
-
+```rust,edition2018
 use nalgebra::DMatrix;
 
 fn main() -> Result<(), std::io::Error> {