blake2: fixup rustc 1.78 warnings (#580)

Starting 1.78, rust will report the `as_mut_bytes` method is dead code.

blake2/src/as_bytes.rs

@@ -13,19 +13,13 @@ pub unsafe trait Safe {}
 
 pub trait AsBytes {
     fn as_bytes(&self) -> &[u8];
-    fn as_mut_bytes(&mut self) -> &mut [u8];
 }
 
 impl<T: Safe> AsBytes for [T] {
     #[inline]
     fn as_bytes(&self) -> &[u8] {
         unsafe { slice::from_raw_parts(self.as_ptr() as *const u8, mem::size_of_val(self)) }
     }
-
-    #[inline]
-    fn as_mut_bytes(&mut self) -> &mut [u8] {
-        unsafe { slice::from_raw_parts_mut(self.as_mut_ptr() as *mut u8, mem::size_of_val(self)) }
-    }
 }
 
 unsafe impl Safe for u8 {}