Use std::hint::black_box consistently.

This also removes the `#![feature(bench_black_box)]`. This was
stabilized in Rust 1.66 and anyone building benchmarks will be
on that or later (as they previously would have been on nightly).

This also allows building `cargo build --all-targets` on stable
Rust as it no longer dies when hitting the feature addition in
the benchmarks.

benches/core/matrix.rs

@@ -142,7 +142,7 @@ fn iter(bench: &mut criterion::Criterion) {
     bench.bench_function("iter", move |bh| {
         bh.iter(|| {
             for value in a.iter() {
-                criterion::black_box(value);
+                std::hint::black_box(value);
             }
         })
     });
@@ -154,7 +154,7 @@ fn iter_rev(bench: &mut criterion::Criterion) {
     bench.bench_function("iter_rev", move |bh| {
         bh.iter(|| {
             for value in a.iter().rev() {
-                criterion::black_box(value);
+                std::hint::black_box(value);
             }
         })
     });

benches/lib.rs

@@ -1,4 +1,3 @@
-#![feature(bench_black_box)]
 #![allow(unused_macros)]
 
 extern crate nalgebra as na;