Move `Index2D` trait to the matrix module

docs/mathjax.html

@@ -7,5 +7,5 @@
 </script>
 
 <script type="text/javascript" async
-        src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.7/MathJax.js?config=TeX-MML-AM_CHTML">
+        src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.7/MathJax.js?config=AM_CHTML">
 </script>

src/lib.rs

@@ -1,7 +1,6 @@
 extern crate core;
 
 pub mod decompose;
-pub mod index;
 mod matrix;
 mod util;
 

src/matrix/mod.rs

@@ -1,4 +1,4 @@
-use crate::index::Index2D;
+use index::Index2D;
 
 use num_traits::{NumOps, One, Zero};
 use std::fmt::Debug;
@@ -7,6 +7,7 @@ use std::iter::{zip, Flatten, Product, Sum};
 use std::ops::{Add, AddAssign, Deref, DerefMut, Index, IndexMut, Mul, MulAssign, Neg};
 
 pub mod ops;
+mod index;
 
 /// A 2D array of values which can be operated upon.
 ///