core: check pointer equality when comparing byte slices

src/libcore/slice.rs

@@ -1831,6 +1831,9 @@ impl<A> SlicePartialEq<A> for [A]
         if self.len() != other.len() {
             return false;
         }
+        if self.as_ptr() == other.as_ptr() {
+            return true;
+        }
         unsafe {
             let size = mem::size_of_val(self);
             memcmp(self.as_ptr() as *const u8,