HashWriter for digest::Update (ex. hmac) #1555

digest_io::HashWriter requires the digest implement Digest. However, the hmac crate's Hmac and related types only implement Update (likely because they would be unable to sanely implement Digest::new, since they need a key to work with).

A version of HashWriter that works with Update instead of Digest would be help with those types, as Update is the only interface it really needs after creation.