AEAD: Remove all use of ring::endian and u64 from Block.

In particular, eliminate the use of `ArrayEncoding::as_byte_array` as
we work towards removing that function because it uses `unsafe`.

Where `Block` is used in parameters to C/assembly code, I verified that
the C/assembly code uses `uint8_t *` as the function type (meaning
`uint8_t[16]`) in the BoringSSL headers. Until recently the stuff in
GCM was using `uint64_t` or a union containing `uint64_t`, which is why
block was previously defined in terms of `BigEndian<u64>`.

src/aead/block.rs

@@ -12,19 +12,18 @@
 // OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
 // CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 
-use crate::{endian::*, polyfill::ChunksFixed};
 use core::ops::{BitXor, BitXorAssign};
 
 #[repr(transparent)]
 #[derive(Copy, Clone)]
-pub struct Block([BigEndian<u64>; 2]);
+pub struct Block([u8; 16]);
 
 pub const BLOCK_LEN: usize = 16;
 
 impl Block {
     #[inline]
     pub fn zero() -> Self {
-        Self([Encoding::ZERO; 2])
+        Self([0; 16])
     }
 
     #[inline]
@@ -45,6 +44,8 @@ impl Block {
 impl BitXorAssign for Block {
     #[inline]
     fn bitxor_assign(&mut self, a: Self) {
+        // Relies heavily on optimizer to optimize this into word- or vector-
+        // level XOR.
         for (r, a) in self.0.iter_mut().zip(a.0.iter()) {
             *r ^= *a;
         }
@@ -65,14 +66,13 @@ impl BitXor for Block {
 impl From<&'_ [u8; BLOCK_LEN]> for Block {
     #[inline]
     fn from(bytes: &[u8; BLOCK_LEN]) -> Self {
-        let bytes: &[[u8; BLOCK_LEN / 2]; 2] = bytes.chunks_fixed();
-        Self(bytes.map(Into::into))
+        Self(*bytes)
     }
 }
 
 impl AsRef<[u8; BLOCK_LEN]> for Block {
     #[inline]
     fn as_ref(&self) -> &[u8; BLOCK_LEN] {
-        self.0.as_byte_array()
+        &self.0
     }
 }

src/endian.rs

@@ -22,16 +22,6 @@ macro_rules! define_endian {
         #[derive(Clone, Copy)]
         #[repr(transparent)]
         pub struct $endian<T>(T);
-
-        impl<T> core::ops::BitXorAssign for $endian<T>
-        where
-            T: core::ops::BitXorAssign,
-        {
-            #[inline(always)]
-            fn bitxor_assign(&mut self, a: Self) {
-                self.0 ^= a.0;
-            }
-        }
     };
 }