BIP 345: Fix OP_VAULT_RECOVER specification for the recovery-sPK-hash

The recovery scriptPubKey needs to be prefixed with its CompactSize-encoded length when computing the sPK hash.

I verified this by:

• Checking the functional tests, where recovery_spk_tagged_hash() is implemented using ser_string() which adds the CompactSize-encoded length prefix
• Using an implementation in Minsc to construct a transaction spending using OP_VAULT_RECOVER and broadcasting it to mutinynet (see line 2 for the recovery sPK hash computation)
• Checking with @jamesob

Alternatively, the implementation could be amended to match the current specification and drop the length prefix. Since this is committing to a single element, the length isn't really necessary.