Run cargo clippy --fix (#250)

* Run `cargo clippy --fix`
* Run `cargo clippy --all-features --fix`
* Fix other clippy warnings
* cargo fmt

Co-authored-by: Luis Moreno <morenol@users.noreply.github.com>

src/neighbors/knn_classifier.rs

@@ -236,8 +236,7 @@ impl<TX: Number, TY: Number + Ord, X: Array2<TX>, Y: Array1<TY>, D: Distance<Vec
 
         if x_n != y_n {
             return Err(Failed::fit(&format!(
-                "Size of x should equal size of y; |x|=[{}], |y|=[{}]",
-                x_n, y_n
+                "Size of x should equal size of y; |x|=[{x_n}], |y|=[{y_n}]"
             )));
         }
 

src/neighbors/knn_regressor.rs

@@ -224,8 +224,7 @@ impl<TX: Number, TY: Number, X: Array2<TX>, Y: Array1<TY>, D: Distance<Vec<TX>>>
 
         if x_n != y_n {
             return Err(Failed::fit(&format!(
-                "Size of x should equal size of y; |x|=[{}], |y|=[{}]",
-                x_n, y_n
+                "Size of x should equal size of y; |x|=[{x_n}], |y|=[{y_n}]"
             )));
         }