Merge branch 'common-fixes' into rayon

Cargo.toml

@@ -2,6 +2,7 @@
 name = "mandelbrot"
 version = "0.2.0"
 authors = ["Jim Blandy <jimb@red-bean.com>"]
+edition = "2018"
 
 [dependencies]
 num = "0.1.34"

src/main.rs

@@ -1,4 +1,3 @@
-extern crate num;
 use num::Complex;
 
 extern crate rayon;
@@ -130,8 +129,6 @@ fn render(pixels: &mut [u8],
     }
 }
 
-extern crate image;
-
 use image::ColorType;
 use image::png::PNGEncoder;
 use std::fs::File;