Remove BlackBox type

Depends on: dalek-cryptography#149

At the time this provided an interesting way to use the `subtle`
implementation of `black_box`, but now that `core::hint::black_box`
is stable someone who wants equivalent functionality can implement it
themselves using that.

Author: tarcieri

src/lib.rs

@@ -950,21 +950,3 @@ impl ConstantTimeLess for cmp::Ordering {
         (a as u8).ct_lt(&(b as u8))
     }
 }
-
-/// Wrapper type which implements an optimization barrier for all accesses.
-#[derive(Clone, Copy, Debug)]
-pub struct BlackBox<T: Copy>(T);
-
-impl<T: Copy> BlackBox<T> {
-    /// Constructs a new instance of `BlackBox` which will wrap the specified value.
-    ///
-    /// All access to the inner value will be mediated by a `black_box` optimization barrier.
-    pub fn new(value: T) -> Self {
-        Self(value)
-    }
-
-    /// Read the inner value, applying an optimization barrier on access.
-    pub fn get(self) -> T {
-        black_box(self.0)
-    }
-}

tests/mod.rs

@@ -487,10 +487,3 @@ fn less_than_ordering() {
         1
     );
 }
-
-#[test]
-fn black_box_round_trip() {
-    let n = 42u64;
-    let black_box = BlackBox::new(n);
-    assert_eq!(n, black_box.get());
-}