Fixing Rust 2018 idioms.

tests/constants.rs

@@ -1,5 +1,3 @@
-extern crate sigils;
-
 use std::{f32, f64};
 use sigils::Constants;
 

tests/lib.rs

@@ -1,5 +1,2 @@
-extern crate sigils;
-
-
 mod constants;
 mod vector;

tests/vector.rs

@@ -1,5 +1,3 @@
-extern crate sigils;
-
 use std::ops::{Add, Sub, Mul, Div, Rem};
 use std::ops::{AddAssign, SubAssign, MulAssign, DivAssign, RemAssign};
 use sigils::vector::*;