universal-hash: add UhfBackend::blocks_needed_to_align

Originally suggested by @str4d here:

#965 (comment)

The use case is ensuring unbuffered processing of the input AEAD message
body when the AAD does not line up with the number of blocks it's
processing in parallel.

This number of blocks can be input from the start of the message body to
finish completely filling the buffer, ensuring all subsequent processing
occurs at the start of the buffer.

universal-hash/src/lib.rs

@@ -53,6 +53,13 @@ pub trait UhfBackend: ParBlocksSizeUser {
             self.proc_block(block);
         }
     }
+
+    /// Returns the number of blocks that should be passed to `Self::proc_block` before
+    /// `Self::proc_par_blocks` can be used efficiently. This is always less than
+    /// `Self::ParBlocksSize`.
+    fn blocks_needed_to_align(&self) -> usize {
+        0
+    }
 }
 
 /// Trait for [`UhfBackend`] users.