add OutputHash::verify

Users could already do this manually, but it's nicer to have a properly
named function that exposes this very common operation.

host/mushroom-verify/src/lib.rs

@@ -204,6 +204,11 @@ impl OutputHash {
             len: output.len() as u64,
         }
     }
+
+    /// Verify that the hash matches the byte slice.
+    pub fn verify(&self, output: &[u8]) -> bool {
+        *self == Self::new(output)
+    }
 }
 
 impl From<OutputHash> for HashedInput {