Use "unfinalized" in header of Midstate

The midstate has not been finalized [0], so use the term in the struct
header.

FTR I don't know _exactly_ what "finalized" means in the context of
sha256 hashing (or hashing in general). This change came from a review
suggestion and we have other mentions of "finalized" in the code.

hashes/src/sha256.rs

@@ -149,7 +149,7 @@ impl Hash {
     pub const fn const_hash(bytes: &[u8]) -> Self { Hash(Midstate::const_hash(bytes, true).bytes) }
 }
 
-/// Output of the SHA256 hash function.
+/// Unfinalized output of the SHA256 hash function.
 ///
 /// The `Midstate` type is obscure and specialized and should not be used unless you are sure of
 /// what you are doing.