Merge rust-bitcoin/rust-bitcoin#3221: feat(bip158): compute canonical filter hash

96e0e720fd feat(bip158): compute canonical filter hash (Rob N)

Pull request description:

  From [BIP-157](https://github.com/bitcoin/bips/blob/master/bip-0157.mediawiki#filter-headers)

  > The canonical hash of a block filter is the double-SHA256 of the serialized filter.

  If a user forgets the "double" in double-SHA256 they will be computing a nonsensical filter hash when this is easily handled by the API.

ACKs for top commit:
  Kixunil:
    ACK 96e0e720fd
  tcharding:
    ACK 96e0e720fd

Tree-SHA512: 5fc0b1632e2327adacbd0ab56b4cd7edce0774f8be2f819782519c51b9691a748f4b3c01887f3bf1146dee49a35f9dfac833f53ae69ee7a53858bd2cedcec01a

bitcoin/src/bip158.rs

@@ -149,6 +149,12 @@ impl BlockFilter {
         FilterHash(sha256d::Hash::hash(&self.content)).filter_header(previous_filter_header)
     }
 
+    /// Computes the canonical hash for the given filter.
+    pub fn filter_hash(&self) -> FilterHash {
+        let hash = sha256d::Hash::hash(&self.content);
+        FilterHash(hash)
+    }
+
     /// Returns true if any query matches against this [`BlockFilter`].
     pub fn match_any<I>(&self, block_hash: BlockHash, query: I) -> Result<bool, Error>
     where