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